Unable to delete selected mails at once in Mail app after iOS 18.2 update

My guess this has something to do with you having Categories selected in your mail, not List view, and as such when you select a mail category, it will remove everything in that category as shown in your screenshot. Take a look at this support document about deleting mails. It has more information regarding Priority Mail. Delete emails on your iPhone or iPad - Apple Support

I just tried on my iPhone 16 Pro Max, tap Select, select the mail you wish to delete, then go to the bottom right corner and select Trash.

Were you able to do that before iOS 18? Probably no. You have an option to select all and delete.

When I try to delete a single email, it only lets me delete all emails from the same sender after update to iOS 18.2. I can only trash all or none. Very bad update