OH doesn’t actually know if the dimmer is operating. It has no way to know (there is no heartbeat or anything sent from the switch). The ONLINE means that the configuration (in OH) not really anything else.
I agree with @denominator I’d first try “rebooting” the switch by killing the breaker or pulling the reset switch open and back on. I actually had to do this with a dimmer of mine the other day. If that didn’t work I’d next try factory resetting/relinking the dimmer and trying again.
Pulling the reset just reboots. If you pull the reset, then push/hold it in until it beeps it factory resets the dimmer deleting its link table. Not sure about double entries in the PLM, but probably not and even if it did, it is pretty unlikely to have any effect. You can always use insteon-terminal to check/delete as needed.
I’ve had weird stuff happen over the years where a switch stops responding to things and when i check it with insteon-terminal I’ll find one of the links missing in the link DB or a weird setting in one of the flags. I’ve wondered if from time to time bits get flipped in the internal memory. Either resetting/relinking or using insteon-terminal to fix my link db / flag setting has always fixed the issues.
@ranielsen - is there a way these days to see a devices flags through your binding in the OH console as to not have to install / understand insteon-terminal? Maybe @InTheDark can check his link DB and flags that way.
Insteon Dimmers are great until they are not. Purchased a number of them for my home but after 10 years they started failing. Got a good deal back then during a holiday sale but now having to replace them makes it seem like a pricy endeavor.