Flole

Well you could run dtdiff on those files and it should list all changes between those 2 versions. If it's only a single change that's it, if there are multiple ones it's a little trial and error to figure out which one caused it.

If you want to investigate further you could decompile the dtb and then compare the 2 dts files to see what changed. It shouldn't be too much I guess.

Using the new kernel with the old dtb's works fine. So the issue is likely part of one of the dtb files.

There is an issue in recent u-boot versions that causes this behaviour. The commit that introduced it is a9bf024b2933bba0e23038892970a18b72dfaeb4. A possible solution is to disable all the EFI-stuff in u-boot, then it boots perfectly fine in recent u-boot versions aswell. So if armbian wants to update u-boot for this board, keep this in mind. I have reported this to u-boot aswell so hopefully it will be fixed at some point there.