How odd – but great you found a fix.
I can still involve one of our devs but realistically if this is not a problem going forward (ie, when you create new events) it does rather suggest a historical issue, whether caused by a conflict with some other code or something else, and I don’t know that we would be particularly better placed to fix that retroactively than you are.
My feeling is that updating them manually, though I fully appreciate that could be something of a chore, is probably the most expeditious way to solve this for you and your client.