Cannot bulk trash messages

Phone is LG G7. Since the latest K9 update (at least) I have not been able to send messages to trash by selecting “Select All” and hitting the trash can symbol. “Select All” seems to function okay, but when I hit “trash”, the phone seems to just sit there, sometimes closing down K9 after awhile.

The number of messages has grown quite unwieldy, and clicking each one and deleting is beyond tedious. But once messages have reached the trash, the “Empty Trash” command still works.