.Bizzare video output - C236 WSI + E3-1235Lv5

Hi!
To be honest, I have a bit weird issue and I don't know if it is hardware or software related.
The video output on C236 WSI from integrated GPU in Xeon E3-1235Lv5 is present, but only if I make full shutdown of XigmaNAS(OS based on FreeBSD 12.1) and replug power cable to the NAS. Otherwise, if I i.e. make a normal restart, the video output will go blank and display will go to sleep mode.

When I replug the cable, the system is booting up for brief moment, then it shuts down and then it boots up again with video output.

My hardware spec is also a bit non-standard:

AsRock C236 WSI
Xeon E3-1235Lv5
2x16GB DDR4 ECC
Bifurcated PCIe 16x into two PCIe 8x:
1. Mellanox ConnectX-3
2. NVMe adapter for Corsair MP500 120GB (L2ARC)
8xWD RED 3TB in raidz2 - they are using all SATA ports from the board.
SATA M.2 Drive in USB enclousure connected to internal USB3 port as a boot device (Embedded XigmaNAS - boot to ramdisk) - I can only boot from USBs, all other ports (SATA/NVMe/PCIe) are occupied.

I really would not like to mess with untwisting the hardware for two reasons:
1. It is my working server
2. It was hard as hell to put all together into U-NAS NSC-800. The hardest and tightest build I ever made.

Currently I have 3 guesses what could cause the issue.

1. Maybe FlexBoot from Mellanox 10Gbe card is interfering?
2. Some change in FreeBSD - if I remember correctly, I did not have this issue some time ago - however I can not tell on which version, since my uptime are pretty high.
3. Maybe USB3 enclosure for M.2 drive is causing issues. It's not very typical boot drive. I'm using it, since normal USB-sticks have died in the past.

Do you have any advises for troubleshooting?

Edited by ASRock_TSD - 13 Apr 2020 at 3:35pm