If the PLC is waiting for you to turn it off, and that is the only logic remaining, the PLC can turn it back on before OPC polling sees that you succeeded turning it off.
If the process in the PLC is complex, natural delays might have the race only show up when comms are disturbed, so you might have never experienced it. And then it happens and you can’t explain it, because you don’t believe what you are doing is “wrong”.