I have been doing something similar with the 5V output, though not battery powered. Until now my only finding is, the Board boots every time, as long as the PSU has been powered on about 1s before connecting 5v to the board. Thats why I assumed there might be an incompatibility with this type of PSU.
I might try with batteries to see if it has any effect, I assume it will boot just fine.
Second PSU didn’t help. As this was the same type and vendor, just with slightly different specs I now have ordered yet a third PSU which also allows 5v voltage regulation. I have also ordered another Board from China to double check if theres an issue with the particular one I am using. But I’ll have to wait a few weeks for the board to arrive.