In our workflow, most of transitions are restricted to the assignee (only the guy assigned to the issue may request transiting to next step in the workflow). This means that the transition button does not offer the same list of transition depending on the assignee.
Until a few fays, assigning an issue to yourself was updating the transition button in real time, and you could apply a transition that is normally restricted to the assignee.
But for the last few days, you have to refresh the page for the button to take into account the new assignee.
Example:
1- create a new issue
= the new issue is not assigned by default in our workflow
= the transition button offers only one possible transition
2- assign the issue to yourself
= you should now be allowed to transit to step A, but the transition button still prevent it
3- refresh the page
= now the transition button offer the transition to step A according to the fact that you are assigned to the issue
A few days ago, step #3 was not necessary.
I am using Chrome 85.0.4183.102, with Jira (on cloud).