These new MacBooks have mini LED type displays, and have more dimming zones (small areas where the backlight can be regulated independent of other areas). I am thinking it's possible there's something up with the backlight related to the new board.
I am not super familiar with the boards on these because they are so new and I would be much more likely to think this is related to the display. But if it is occurring with different displays, that seems to rule that out. I suppose its possible there's something to this related to pairing to the logic board, as Apple is wont to do. But I have no practical evidence for that.
In light of some info coming in now that Apple's Self Repair is live for MacBooks, it appears this is related to "Calibration" performed by Apple. Basically, the screen (and Lid Angle Sensor) are paired to the logic board, which enables whatever Mini-LED magic the display is doing. So if your display and logic board are not paired together with Apple's software, you're likely to get this issue.
I continue to be perplexed as to why this serialized pairing is necessary for proper function, but that's a whole different discussion.
As info continues to come in about repairing these models, here's the latest on what the independent repair community is finding. The TCON Board on these is underneath the bezel at the bottom of the display rather than attached to the cables dangling from the screen.
If you're replacing the display and not using Apple's Self Repair program where System Configuration is accessible, there are two ICs (Integrated Circuits, or more commonly - chips) that need to be transplanted from the original TCON board to the new one to maintain the configuration. This not really a viable fix for most DIYers, since it requires the skills and tools desolder and solder BGA components, but it's how people outside Apple's authorized repair ecosystem are resolving this issue. I will try and find some pictures for reference, but I don't have any of these in my spare parts bin yet.
Did you replace the display assembly?