I also have ha this issue for several months know, using 2019 12.9 IP-Pro and Smart Keyboard. It’s really annoying, as i then default to either using my iMac or iPhone to delete multiple junk emails.
In the past [prior to 14.2] a work-around was to type text in the search and then delete using the delete key. After this i could highlight multiple emails anew then delete. Using 2finger gestures is no where near as quick as using the keyboard.
I’m glad ios14.3 fixes this, but i think apple needs to send out a fix asap