That's actually a good point, i'll strip the boot.img from an older LOS 15.1 rom to test if the upstreaming was the problem. For completeness sake, the kernel versions i am currently testing are as follows:
mk2000 & Swan -> v.3.18.140
LOS 18.1/Lighthouse Stock -> v3.18.113
Gamma Oreo Stable(the one i just confirmed to have the IR working, gamma-pie throws many errors here) -> V3.18.71
So yes, the Gamma Kernel is running an older version of 3.18 and the IR works fine there. That leaves us with 2 things: Either the kernel upstream broke the IR, which i will verify shortly, or Gamma has some commits that fix it when compared to Lineage's sources like the CAF upstream i linked some time ago. I will also run a git log on lineage's commit history to check if there is anything suspicious after i test the old lineage kernel. As it stands, only gamma managed to get the IR working on my H910.
Small edit: Will be testing the lineage 15.0 kernel source instead since 15.1 also seems to have been updated do 3.18.113. At least on github, LOS 15.0's branch appears to be running on v3.18.78 which is the closest one yet to gamma.
Edit 2: Yeah... things aren't looking good. I couldn't find a LOS 15.0 kernel or even a 15.1 for that matter(x86cpu's requires drive access and i'm not even sure he will answer) and also didn't manage to compile a working one as well. Not only that, but the lineage's sources definitely had the IR broke as far back as 3.18.102 at least, which is the oldest one i managed to compile with some changes and a supposed IR fix committed in the same code base
here. All newer LOS kernels should already have that fix, and that means that this fix itself does not help much. Even compiling the kernel after reverting it to this exact commit results in a non-working IR. To be honest, i don't know what to do now because all commits related to the IR are either newer than 3.18.102, or do not fix the problem. If anyone else has a tip or anything that may help solve that i'm all ears.