So I haven’t exactly nailed down what causes this bug. But here’s what it seems to be:
On some actions that require holding Shift (like clearing the tape), upon completing the action, the device can think that shift is still pressed.
It’s as if the OP-1 registers shift as unpressed only when the key is released, not based on the actual state of the switch. I.e. it misses the key release during the tape erase (or whatever other action is involved).
Again, I haven’t figured out exactly when it happens, so I can’t avoid it, but it can have undesirable consequences. For example, if you hit the synth button after the bug occurs, you lose the current synth settings (if you hadn’t saved them).