You are correct, I can reproduce the bug on 7.10.10.
I think this has something to do with the fact that the email field is a bit special, the field is called email1 in the database, but it’s just a proxy for the primary email. The actual emails are saved as a one-to-many relationship, so that each record can have more than one email.
Some of that logic seems to be faulty when you change the field to “required”. Note that there is no way to change relationships to “required”, which would be the thing to try next. So I am not sure we can really solve this easily.
You can try working around this by adding some validation onto the view’s javascript (requires coding):
I shall pass this to a more experienced developer here and will raise it in GitHub
At least the way the client is using it means that an Adviser HAS to edit the Account and Contact once converted and that is then enforced.