I might be misunderstanding your issue, but it seems to work fine for me. I pasted a screenshot into a comment and you can see it in the card in the background:
Then I deleted the “image.png” file by hovering over it in the Attachments section (under the Project section denoted by the paperclip) and clicking the “X”:
I’ve had team members try to delete attachments by deleting the comment where the image is displayed in line with other comments rather than by removing it from the attachments section as shown above. This section gets overlooked and the fact you have to hover over the file to see the “X” I think compounds this somewhat.
Here you can see the image is removed after i click the “X”:
Another thing I can think of is duplicate images in the task. I have this happen all the time with forwarded emails. If there’s anything listed in the attachments section it’ll pick something.
Lastly, you can also remove an image on a card (this doesn’t really address your issue, it’s just some extra info):
Hopefully something here helps you. If not I’d agree you have stumbled onto a bug I haven’t seen myself yet.