Re-reviewing a previously rejected article but some authors have changed from the previous submission. Should I mention that to the editor?

It's not particularly unusual for authors to be added during a revision of a paper, but for them to be removed is quite unusual and often linked with something improper going on.

Since this submission is a different publication than the original version that you saw, the editors do not have this history in front of them, and I think that it is indeed a good idea to flag this as a concern to the editors.

Moreover, if the article has not been significantly revised but the set of authors has changed, then that's a major red flag that the authors are doing something improper. If this is the case, then the editors absolutely must know, and should probably investigate wrongdoing.

I have been in a situation where a co-author's organization told us to remove them from a paper for inscrutable legal/security reasons. The author gave us permission to remove them. So this not necessarily unethical. But it's a good idea to ask. I would also check if there are changes in the acknowledgements. I think the author would be obliged to thank them for their contributions even if they can't do it by name.

Our co-author's organization eventually let them be re-added to the paper. But I can see how that might look sketchy to someone who didn't have the context.