Follow the Installation Manual instructions, and put 3.74K resistors on the lower zones. There's a reason it directs you to use 3.74K resistors.
I don't know what tests you ran to ensure it's "working", but I assume everything "worked" as expected---but it doesn't matter. A zone can "work" when its resistance is 'way more or less than it's supposed to be, and manufacturers won't tell you what the tolerances may be, and for good reason: Installers aren't supposed to be trying to see how much they can get away with, they're supposed to be installing the resistance values prescribed in the manuals.
One memorable example of this comes to mind, from years ago: A couple of installers upgraded a home system from one panel that used 3K zone loop end-of-line resistors (EOLRs), to a panel that used 2K EOLRs. Now, it's not unusual for the "EOLRs"--i.e., the "End of Line Resistors" that the manufacturer says should go at the end of the zone loop--it's not unusual to install the burg(lary) zone EOLRs on the Panel, the main PCB board, at the beginning of the zone loop.
(There is difference of opinion in the alarm tech community whether this is acceptable practice, but I'm in the camp of installing home alarm burg zone EOLRs at the panel, for reasons I'll discuss with anybody who's interested; but it's not relevant here.)
So the installers switched out the Control Panel and keypads, including switching out the 3K resistors at the Panel for 2K resistors, as prescribed---but the reason I mentioned the burg EOLRs at the panel is that Fire Zone EOLRs NEVER go anywhere except at the End of the Line--the last fire sensor (smoke sensor or heat sensor) on the fire loop. That is not disputed among techs, and it's illegal according to Fire Code to install it anywhere else. It would be worth prison time for me to put it at the panel.
But switching out the 3K on the fire zone for a 2K would have cost the installers a lot of extra time and effort, because the smoke sensor at the end of the line was high above a narrow landing three flights up a staircase. The rest of the upgrade had been in comfy eye-level conditions,but switching out the Fire EOLR would entail wrestling a tall (15') stepladder up 3 flights of stairs, setting it up on a precarious landing (if you've ever stood on a tall stepladder where you're looking down 3 stories, you'll know what I mean.) and having your buddy steady the ladder while you do the switch. He better be someone you trust.
But--oh, joy!-- When they connected the fire zone with the existing 3K resistor and powered up the panel, it indicated that it was 'happy' and Ready to Arm (green light). The Keypad showed no trouble on the Fire Zone! I.e., it "worked." The new panel tolerated it. So they packed up and left. Job finished.
The reason I know about that job is that I'm the service tech who was sent there three days later to figure out why the system had had a false Fire Trouble Alert in the middle of the night before, and now the fire zone wouldn't reset, even though there was no smoke. I found the cause pretty quickly, and I had to haul that tall stepladder up those steps, set it as stable as I could get it on that narrow landing (which wasn't wide enough to open it fully), and climb up it with no buddy to steady it (installers tend to work in pairs; service techs usually work alone.) Fortunately, I climbed a lot of trees as a kid and did some sport climbing when I was older; heights don't bother me, and I'm pretty steady on a ladder, even when I have to have my head craning up looking at the smoke sensor I'm working with. For awhile I kept the 3K resistor for a souvenir.
I don't usually tell 'war stories' except when talking shop with other techs, but I want to impress you with how tolerant a system panel can be about EOLRs---until it isn't. That panel--it was an Ademco--tolerated a 50% higher-than-prescribed resistance on its fire zone for TWO! DAYS! before it decided to object to it. This isn't a flaw in system design, IMHO. Alarm systems have to have a certain tolerance, because the environments of the various zone loops can sometimes throw a lot of transient fluctuations at them. (That's a lot of other stories that I'm happy to drone on about if anybody's interested.)
The zones have prescribed resistances that the panel monitor--with your panel, it's 3.3K Ohms for single zone, and you could probably get away with a 3K or 3.6K if that's all you have---but switch it out for the proper resistor as soon as you can. Yes, it will "work" with an over/under value, but it won't really be happy with it, and will be less tolerant of other transient conditions the loop may encounter, e.g., a nearby lightning strike.
Outside of Zone Doubling, a panel "looks" for either an Open or a Short on the loop--either 'infinite' resistance or 'zero' resistance. It's only with Doubled Zones that it discriminates between set resistances other than Open or Zero. And under certain conditions--like if you have a flaky connection that pushes the limits of tolerance--it can get confused which of the doubled zones has a problem. I recommend you do a good job soldering any and all splices and be meticulous about making tight connections at screw terminals and such. And use the resistors prescribed!
PS: For what it's worth, I'd recommend you put your 'EOLRs' at your NX-8V2 panel--it makes any future troubleshooting a lot easier. Also, just to let you know, your Interlogix panel has recently (2019) been 'orphaned'---i.e., the company that made it decided not to stay in the alarm business. So you (or someone) will be looking at eventually replacing it with another brand, which will probably entail switching out the EOLRs. Which is a major reason I'm in the at-the-Panel school of opinion, for home systems.