Time Machine playing up again

I am running the latest Catalina OS on a 1 year old Mac Mini. I have had several problems with Time Machine, which I rely upon, most in connection with Mac Mail. (I have also had many problems with Mac Mail too, but I don't suppose the two are related.)

Almost a year ago I was unable to recover my archived emails. Now the problem is occurring again.

I have set TM to archive all files (not software). If I open my Documents folder (for example) then enter TM, the back button displays and I can move back in time and see all the files in Documents.

But if I open Mac mail and enter TM, a window comes up with the number of files in that email Inbox shown at the top, but nothing else appears in the window. The back button is greyed out, and I can do nothing but return.

But if I display hidden files, enter backup.backupdb and chose a date, the emails are all there. (I actually can't verify they are ALL there, but many of them are and I presume it is all.)

Thus if I want to recover an old email, I cannot do it except by searching through the strange and difficult file system used by Mail.

Is this a common problem? Is there a solution please?