This was just introduced with 3.1.9
when you switch from upload avatar to gravatar or remote upload and you checkmark for delete avatar it triggers the min max and you cannot delete unless you go back to the upload avatar, which in that case works as all the other forms are disabled.
This is really a double edge bug though. If you try to submit the gravatar or remote upload you trigger the min max as well and cannot continue. The user should not need to enter the size necessarily . As a matter of fact there is the automatic size detection in place for that very reason. This is just a major pain for the UX. Leaves the users wondering why the form is not going through.
It can be fixed if the value is empty where there is no gravatar or remote avatar yet.