Last night, Ms. TTK and I watched Trouble Man on Plex. I don't think we had ever seen the film, although we've had the soundtrack album by Marvin Gaye forever. From the beginning, I suspected that something was amiss, and as the film went on, I knew I was right: All of the scenes in which Marvin Gaye's music was prominently featured used replacement music by what sounded like a bad garage band trying to play funk for the first time.
Interestingly, the movie retained Gaye's instrumental dramatic underscore, presumably because 1) it would have been difficult to remove under the dialog, and 2) this music had not been released on the original soundtrack LP.
All I can assume is that this film somehow slipped into the public domain, but they could not use music that had appeared on the original LP because it was still under copyright. If they had at least indicated this on a card at the beginning of the film, it would have saved us an hour-and-a-half of our lives.
Yet one more reason that many of us prefer the physical object over streaming. Now I can't help but wonder how common or rare this practice is.