Mac Mail Smart Mailbox Not Deleting Messages

Hi

I have Mac Mail set up perfectly with Gmail for business, deletes email as expected and has been working for a long time.

I am starting to use Smart Mailboxes and have it set up perfectly for me, however when I delete an email from the Smart Mailbox it doesn't actually seem to delete properly.

Here's a video of this in action - I delete an email, nothing then really happens for 1 minute and 30 seconds, then at the end you can see the emails I've deleted come back:

https://youtu.be/O1To9ZMHKqU

This also happens if I delete an email from the Smart Mailbox and then "Get All New Mail" as the old emails come back.

To emphasise it doesn't do this when I delete it from the regular inbox, that's set up and works as expected.

Can anyone give me some insight as to how I might start to fix this?

Thanks

Andy