Using the plug that came with the pan
Depending on a number of variables like where the pan and its parts originated from you may have a tolerance stack up issue or poor QC.
I would suggest trying a longer GOOD QUALITY 1/2-20 bolt to see how snug or loose it is in the pan threads. If it's too loose you could go to an oversize plug to resolve the issue. I don't like suggesting that but something is not right and it needs to be identified and remedied.
Could be metric threads in the pan or plug, sometimes the two sizes are real close. Just had that problem replacing blade bolts on my finish mower. Thought they were 5/8 “ course but they were 16 mm. If it wasn’t for the nut and bolt guides on top of the bolt cabinet I wouldn’t have noticed the difference.
it could be either the plug itself or the threads in the pan or possibly both
I have run into this a few times as well along with other QC issues.
TO the OP: FYI 20 TPI is real close to 1.25mm TPI which is 20.32 TPI.
A 12mm bolt is about .472 and a 12.5mm is .492. the 12.5 is a bit uncommon but available in a bolt. Not sure on the drain plug though.
