Unfortunately I think that you have the problem where the assignment is in a state of limbo. The conversation with Amos is "complete" (and can't be retriggered) and yet the assignment itself, as far as the UI and chapter completion is concerned, is incomplete.
If you can wait, then try again after waiting a while and see if it resets (maybe after an update). Otherwise only support can actually fix the bug, although we should try every other option before that because the chance of a ban is very likely.
While there is a very high chance you would get banned, it isn't 100% if you can cast reasonable doubt (although this is a last case resort as your account is in year 7). If you have spent real money on the game at any point they might just fix the problem without looking any closer. Otherwise, you would need the account to be at least 2-3ish years old for you to be in year 7 without spending money.
For example, I actually contacted support on my "main" account over a year ago and I still have the account now (It's caught up in year 8/Hogwarts Beyond and still not banned). I contacted support so that they would add the signed Dumbledore card that is bugged, and they did add it to my account without any fuss. I had been using the modded apk for about 2 weeks at that point, and had spent only £5 on an outfit at one point. It's worth noting that I had made the account a few years before that but quit in year 2, and I resumed playing again last January (so the account was a few years old). It was only in year 3 at that point and I wasn't concerned about loosing it, and yet support didn't ban me.
