I was also trying the figure out the same thing. Something changed and I can’t figure out how to undo it. I need to be able to resolve other people’s comments, as was possible until recently.
An update for anyone following the thread: I got in touch with Figma support and they confirmed this is an intentional update. If you’re a viewer rather than an editor, you can’t resolve other comments “to make sure that viewers can’t accidentally remove other people’s feedback, or important comments that the file owner wanted to have visible”.
They said it should still be possible to resolve comments via the API if anyone needs a workaround.