This question is the same as stories on next-gen projects not crossed out when "done" but the answer in that one doesn't apply.
The answer to that was that they had added a column to their board to the right of "done" and so moving things to done was no longer setting the resolution.
But we have a board that has "done" at the far right. So that's not the problem.
The other answer I found was about Service Desk and Next Gen but this is software.
All the issues that are resolved were resolved on or before the 2020-02-27; all the unresolved issues were resolved after that date.
In the logs there was an event on the 28th but it was removing a user from the project and I can't see how that should affect this at all.
I tried moving an issue from "done" to "in progress" and then back again but it didn't help.