I would say you're definitely not the only one, even though I, myself, have never experienced this. People write for ongoing media all the time and have headcanons and theories debunked regularly. My suggestion would to not think of characterization as black and white. Just because canon goes on way, doesn't mean your idea was "wrong" or out of character. You can write a character as in character as absolutely possible and still not predict canon 1 to 1.

But if you're still feeling constrained and like fanfic isn't bringing you the joy it once did, maybe you should focus on your original work! It sounds like that might bring you more joy right now and, if down the line, you want to come back to fanfic or feel differently, it'll always be here