With the merge of 397faf6 (a commit which belongs to the setup_improvements branch) completely regressed the USETUP module which prevents ReactOS from proceeding with the next stage of the setup anymore.
USETUP installs the bootcode of BTRFS/FAT successfully but after the completion of the 1st stage, within the screen it's displayed "Partition signature != 55AA" if you chose BTRFS, meanwhile in FAT it doesn't display anything.
A debug log is attached with two screenshots explaining the issue which I described above.
NOTE: It looks like the regression doesn't occur when you have a newly unused partition (for example, when you create a new VM). To get a better chance of reproducibility, the partition has to be already formatted before with an existing FS on it.
For this reason I'll reduce the priority level to critical since you can still somehow ReactOS.