Working in a form - then suddenly it changes to the background.

One more to add to my list of strange behaviour in 12.1.3:

Users are reporting that they navigate to a form (various responsibilities) and drill down to an entry or query sub-form. They enter or query on numerous records and "poof!" the sub-form moves to the background all by itself. The user then clicks on the sub-form to bring it back to the foreground - and field focus is on the first record, meaning the user has to navigate back to the record that was being entered/viewed when the sub-form flipped to the background.

Any ideas to fix this?

Hal