Can't delete keychain items with UI, but command line works fine
I notice you say, "If I now redo the search in Keychain Access…" implying that you began by searching when you weren't able to delete the item. Find it in the list without searching and then delete. It's just a dumb bug. I have found I sometimes can delete a single item after searching, but not multiple, and sometimes there's no problem at all. ¯\_(ツ)_/¯