If this hobby project is merely a mental-masturbation exercise, then it matters little whether you isolate and repair the fault in question.

If, however, you want to produce a properly functioning circuit, meaning that you want to do things properly, then you must determine what it is, under the circumstances that you've now determined triggers this faulty behavior, causes the MCU to behave as it does. Once you've determined that, you can proceed to effect repairs, whether in hardware or in firmware.

You've determined that this apparent fault becomes evident when you turn off the input power. Why does it do that? What means are available to inhibit this faulty behavior?

I'd recommend you determine why it happens before going off and attempting to apply a fix.

RE