Messages keep showing as unread

I have had an issue with my messages app showing that there are unread messages, despite none being visible in the app. Found a solution and posting as I found no viable solution elsewhere. There was a previous thread on this where the top suggestion was to ask siri to read all unread messages - this would work great if it was a few, but I had hundreds.

No amount of setting changes or restarts fixed it, but I have found a new solution! Choose select messages, but instead of ‘mark all as read’ drag your finger to mark the messages from the top all the way to the bottom of the screen - screen will keep running and all msgs can be selected. Now ‘mark an UNREAD’. (Screen will freeze for a few seconds)

You can now select all and mark as read. Voila.