That looks like an issue. In the screenshot there should be a share icon and a delete dustbin icon next to the three invisible dots! 
I notice the same omission on LineageOS 18.1 as well.
As a workaround go to the corresponding location of the documents on the phone’s storage; you can delete from there.To do that: pull out the menu from the left side of the screen, or select from the hamburger menu at top left of screen and tap on your phone icon at the bottom of the list.
Actually, the problem seems to be just with the Recents selection. Select from Images or Audio on the pull out menu and the dustbin/trash icon is present. Is it the same for you?
