I have decided...

[acroyear]

That the opposite of a Schoedinbug needs a name.

Unusual software bug - Wikipedia, the free encyclopedia:

A schroedinbug is a bug that manifests only after someone reading source code or using the program in an unusual way notices that it never should have worked in the first place, at which point the program promptly stops working for everybody until fixed.

My testers run into the opposite (but it's not quite a Heisenbug) - they hit a problem that repeats itself until I stare at the code and follow every function/method call and determine that there simply is no way that bug could possibly happen...

...and the next run it disappears permanently, without a single change in code or environment.