I’ve done the exact same thing a number of times without that sort of malfunction of a SmartThings plug so it’s definitely not standard behavior.
My only thought other than “It could be totally bolloxed. I hope not.” is that it could have taken a surge and gotten “confused.” You might try unpairing it, doing a hardware reset, and pairing it again if it seems to have recovered.
Sounds to me like the plug may have failed as even if it was set to last power or power on when power is applied, you should still be able to toggle in on / off manually.
Maybe as suggested the only thing you can try is to reset it, if it still behaves this way then I think it probably ducked.
Delete the device from home assistant, plug the plug in, power it on and hold down the button until it starts to flash then try adding it back to home assistant.