MacOS Mail app sort by Date doesn't work

I'm using the most recent MacOS. I use the Apple Mail app for my emails. I sort by Date. But unfortunately over the last few weeks/months the date order is all over the place.

The only way I can get the emails to correctly sort by Date is to sort by something else (attachments for example), then sort by date to get them to sort by date. But after an hour or so the order goes wrong again.

• I've tried rebuilding my mailboxes. This only works for a short time.
• Organise by Conversation is off.
• I'm using POP type email so it is not a synching issue (the emails are downloaded and live on my hard drive).

I suspect it is a software issue