I am getting read only text field and comment field on a transition screen that is opened when trying to change status on an issue selected from a board - so this is a pop-up on top of a pop-up.
Actually the behavior is exactly as described in https://jira.atlassian.com/browse/JRACLOUD-68799.
All goes well if the transition is done from the issue edit screen.
So no field scheme/permission/workflow condition issues.
Should a bug be opened/reopened for Jira?