Could be a bug in the alert conversion process. I have yet to upgrade any of my client instances from NPM 11.0.1 (and earlier in some cases), so I haven't seen anything like this first hand but I would recommend you go through all your alerts if you have not done so to ensure there aren't any others with the same apparent bug.
I also suggest that you log a ticket with support via the customer portal or this link, and have them investigate. It will help them improve NPM 11.5.x for those of us yet to upgrade, and it'll also make sure you get a speedy response!
If you do, could you post any updates from support here please?