Est. Revenue field is locked on Opportunity main form

Hi All,

A few users are reporting that the Est. Revenue occasionally locks on the main Opportunity form when viewed in the web browser.  I don't have any business rules or form scripts that lock that field.  I also looked into the Revenue field and it is always set to "User provided".  I have not been able to replicate this issue, but it keeps happening with a few users.  

A) what could be the causes of this issue?

B) when it happens again, how should I go about investigating/debugging it?

Thanks in advance,

Ben

Current version: 8.2 / Online