Getting air in the lines somehow

IMO, you may be boiling the brake fluid.Or rather some component in the brake fluid,lol.
Do not use shop brake fluid. Invariably it has absorbed moisture from the air. When you heat the brake fluid, that moisture flashes to steam when you release the pedal at the end of the braking run, and there is your air! The confirmation is that you can run around for days with no issues until that one time you heat up the fluid. Another confirmation is that it never happens during testing in the shop, on a cold system.
Drain all the fluid and put in FRESH from a SEALED container,hi-temp, disc-brake fluid. Then, if you haven't already done so, install those "stupid little metal spacers" on the pads between them and the pistons to help divorce them.
I have been using the 15/16 M/C since 1999, on my KH factory system,with silicon fluid,and with zero problems.And I love it.
Good luck