Yeah, that sounds like a hardware issue. Maybe ask in the realtek thread mentioned above, you might reach people with more in-depth knowledge of the platform there.
Maybe there is a linux bootarg, that can disable loading the POE part on bootup. That way, you could continue testing with the POE-variant firmwares. Somebody in this thread, or the other one might know such a bootarg.
Does the POE part work properly when using the vendor kernel? Do you have (enough) POE-capable Hardware to test?
The POE-part recently went through a lot of changes, moving from user-space handling with the realtek-poe package to the PSE-framework backported from recent 7.x kernels.
If you compile your own kernels, you could attempt bisecting, or removing some relevant PRs.
Poe on port 8 does not work with vendor image also while port 5-7 supplies power it does not measured on web page nor there is led lit (actually on openwrt its Poe led lit until kernel crash) , but then since found out non HP openwrt firmware crash with random kernel errors, tried to decrease CPU speed same, maybe it's ram has bad blocks. Was able to put back vendor firmware that seems to work for some reason at least it was running for 2 hours without lockup... But pretty sure it is a hardware issue now. And noticed CPU heatsink hell hot not sure it's suppose to be connected to the case via thermal pads this one not have such thing. 
Please retest a initramfs boot with current snapshot.