Tic If you have already done a factory reset, so there is nothing on the phone you care about, you have room to experiment. Maybe sideloading will work now that there shouldn't be potential interference from the plausibly-bad/now-gone user data partition.
I have a faint memory that you can see the "corrupt" message on startup for a while even if everything is fine, because the procedure that clears the "corrupt" flag runs only in limited circumstances. If sideloading the current OTA image doesn't clear the "corrupt" flag, then I think your options might be (a) flashing back to Google's stack and then reinstalling GrapheneOS from scratch (kind of a pain), or (b) waiting until a new GrapheneOS release is issued, which I suspect will cause the "corrupt" flag to be cleared.
But I am not an expert, so I might not have this all correct.