Today I finished every campaign on pro difficulty in coop mode. Unfortunately I didn't get the achievement for completing the game on veteran and pro difficulty. I used the infinite ammo option but I read that it doesn't block the achievements. So what's the problem?
There is a bug: if you accept an invite to join someone else's game, then you are credited with playing whatever difficulty you last hosted / soloed up to a checkpoint, regardless of the difficulty set by the host.
When my friend and I played on Pro (with me hosting), I got the pro and veteran achievements at the end, he didn't. Checking his records, his chapter completes were all on normal (the last difficulty he'd played as host).
(I also got some Veteran times recorded on Ada's campaign when I joined his "normal" difficulty playthrough as Agent, since I'd been soloing Veteran previously). ---