It seems that the BraiinsOS+ kernal log is different from stock bitmain? I dont see anything about hashboard voltages in there.
I want to be fair to BraiinsOS+, personally, I have not tried to mix and match 17 series hashboards using their firmware, I see people claiming that it's the case and that BraiinsOS doesn't even read the EEPROM but I can't confirm it, however, I have personally tried Vnish to overcome this mix-match issue and I can personally confirmed that it worked.
With that said, how are you sure that it is not fixing the compatibility issue, for all we know, the ones that don't work, might just be dead, to confirm that this isn't the case you are going to have to run them individually, i.e a single hashboard plugged, the only way you can come to a conclusion that the firmware isn't fixing the compatibility issue would be to have 2 (working) hash boards running together, if they don't work smoothly then you know it's a compatibility issue, of course, provided that you know the PSU is good and is capable of running more than 1 hahsboard.
Also, it would be great to comment on their topic, they have a representative here who is very knowledgable in that firmware aspect, and should confirm wether their firmware indeed fixes the issue, if I had to guess, I'd guess it does fix the problem and whatever results you are getting are caused by other issues.