David Harbour Says Millie Bobby Brown Reports Triggered Mental Health Breakdown

David Harbour is opening up about a difficult chapter in his life ... addressing both a mental health struggle and rumors of tension with his "Stranger Things" costar Millie Bobby Brown ... and he says one triggered the other.

David says he suffered a mental breakdown last year that was "confusing as hell" not only for himself, but also for those around him ... telling Variety that depression, mania, and other mental health episodes can be difficult for loved ones to understand in real time.

The "Stranger Things" star admitted he was triggered by what he claims was false reporting on his relationship with Millie ... namely, reports she had complained about his behavior on set and that the pair had experienced friction behind the scenes.

As we reported ... in December, David took a man's hat in a bizarre interaction at a California bar. The incident followed reports that Millie filed a bullying and harassment complaint against him.

The actor is now acknowledging there were disagreements with Millie, but he's comparing their dynamic to that of a real family ... noting they worked together for a decade during some of Millie's most formative years while playing a father-daughter duo on screen.

According to David, they always found their way back to each other -- with Millie echoing that sentiment in an email statement to the publication, explaining that as the years went on, their working relationship became increasingly collaborative and that they often pushed each other emotionally to get the best performances possible.