I can't watch movies which are available on prime video to me. I can see Tropic Thunder on prime video, and confirmed that I can watch it there, but cannot join a watch party watching that movie.

It says the streamer is "watching a title not available in your region", but it is available.

I tried to submit this as a support request, but nobody actually reads what's written and I just got an automated response about hosting watch parties the first time then giving feedback via uservoice this time.

Fix your support form, and watch parties.