David Harbour Won't Attend 'Stranger Things' Finale Event Amid Bizarre Behavior

David Harbour's taking a step back from the spotlight, it seems ... skipping a big "Stranger Things" event after a truly bizarre incident where he allegedly stole a fan's hat.

The actor was initially supposed to attend the "Stranger Things: The Final Season Celebration" at New York City's Paley Center Thursday ... however, he's no longer listed as an attendee on the event's website.

A rep for the Paley Center told People he can't make it due to a "scheduling conflict."

The timing of the cancellation may raise some eyebrows among fans ... given Harbour's seemingly been in the news for all the wrong reasons over the past few months.

As you know ... Harbour's going through a messy divorce with Lily Allen -- who seemingly spilled all the juicy details about their relationship on her recently released album "West End Girl," including alleged sex toys she found which she claims he was using on someone else.

There were also rumors of Millie Bobby Brown filing a harassment report against Harbour for alleged abusive behavior on-set ... though Brown categorically denied the rumors and even lauded Harbour as an actor and collaborator.

And, we broke the story just yesterday ... Harbour was caught on video running down the street with a bar patron's hat in a weird altercation in Encinitas, California. He later threw the hat to the ground, asked the group about the Iraq War and then fell to his knees in prayer.

A source with direct knowledge told us, "David has always been honest about managing his bipolar disorder. Wouldn’t be surprised if that’s what he’s dealing with here after working non-stop all year and having his personal life under public scrutiny. That would be stressful for anyone but even more so for someone who is bipolar."

Worth noting ... Harbour isn't the only major cast member sitting out the event because series star Brown will also reportedly not be at the Paley Center this evening.