If you happen to play through and finally quitting a mission without saving, e.g. because it's not allowed to save, like in the first camp mission, then the RESUME button will still load the last savegame, which might have been created many missions back.
Exceptions are autosaves which are ignored.
Given the case you've unlocked at least up to campaign mission "Bomos" for our repro:
If you then save and quit the "Signal lost" mission again, press RESUME again to eventually re-load the same savegame, another bug becomes apparent; after loading the RESUME checkpoint for the second time, you won't be able to leave the mission anymore.