Does the same issue happen when you start Thunderbird in 'troubleshoot mode'? https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird This is the first diagnostic test to be done in a situation like this one.

I've no idea.

The point of my post was to ask if anyone else has this issue and whether it is now meant to work this way?

Often on forums these days people expect anyone posting to run troubleshooting tools and I'm never sure why?

Why would I spend time trying to debug an issue if it is meant to work that way?

I also thought asking how to get from a beta version to the full version might get a reply. I've no idea how I ever got on the beta version in the first place. Again, I have no idea if this is normal or whether I did something wrong.

Which is the size of this Inbox folder and how many messages does it contain? > R-click on Inbox > Properties > Number of messages & Size on disk....

I've no idea [if this happens in Troubleshoot Mode]. The point of my post was to ask if anyone else has this issue and whether it is now meant to work this way?

It's not.

Often on forums these days people expect anyone posting to run troubleshooting tools and I'm never sure why?

Because we try to solve people's problems, and that often requires additional information based on actions you perform.

I also thought asking how to get from a beta version to the full version might get a reply. I've no idea how I ever got on the beta version in the first place. Again, I have no idea if this is normal or whether I did something wrong.

Single question per post please, because different people are able to answer different questions. In other words, please make a new posting https://support.mozilla.org/en-US/questions/new/thunderbird