I noticed yesterday 13/03/24 that 7 days emails in my in box have disappeared. I have not done anything to cause it and they are not in my trash folder. They were there on the evening of the 12th but by morning they were gone I see that BT has been ignoring other posts about the same problem...
Just read your problem. Me too. Found the answer. Far right corner indicates views. Click and at the bottom under message you will find conversation view. Click and they should all appear again. Don't know why it happens but that's my solution.