Have you tried tapping on the first line of the message rather than the body of the message ? That seems to work for me .
When viewing private messages I can only see a summary of the first view lines, not the complete message. There seems no way to open messages in full. In the same way, if I go into my sent messages I can only see the first few lines. So if I want to refer back to something I wrote I can’t read it in full.
I’m quite techy, but I must be missing something, probably being an idiot and it’s something simple? 😀 If you could hopefully enlighten me 😊. I do use an iPad if that makes a difference, but it shouldn’t really these days.