"I just tried adding an ON ERROR STRTOFILE() immediately before the REQUERY(): the behavior stopped occuring. When I commented it out again, I saw the behavior return."

Bingo. Any wait state or other attempt to debug it will make the code work. If you compile a sample into an EXE, you will get reproducable result almost all of the time. As you enter WAIT states or function calls (which trigger events and thus wait states), you get more variability.