Sounds like a bug to me. Do you have any other monitors to hook up? Maybe Blender is using the “last screen loaded” as its resolution definition. Certainly seems like something Blender would do.
Hmmm… I think I have an older monitor lying around somewhere, I’ll check it out.
Edit: I found a spare monitor, but I cannot plug it in as it uses a VGA connector - for which my graphics card has no support :1 Seems I’ve run into another First World Problem : P