Mail I put in the trash totally disappears.

I'm using an iMac with Big Sur v11.7.9. Recently, for no reason I can see, all email I put in the Trash immediately disappears -- not waiting to delete after one month, as I have it set up. This problem seems to be quite new! I have three different email accounts, and this is happening with them all. However, the items I've trashed do appear in the Trash bin on my iPhone XR and iPad Air. (thank goodness...but I do 99% of my work on the iMac).