HOLY SHIT YOU GUYS I figured it out.
I took apart my other board to try that battery and it worked fine.
Then I realized… The socket slid in further on the battery that works.
The first battery just needs a serious fuckoff push to seat all the way in:
WRONG (there’s a “bumpstop” here that makes it feel like it’s fully seated):
RIGHT:
The antispark connector has two rings on one side down inside of it and it looks like the inner ring is much tighter than the outer one.