This is the sequence as I understand it:

Given user A open a revision R
And user A reply in a thread T
And user B open revision R
And user B reply in thread T
And user B saves R:T

When user A saves

Then user A gets an edit conflict

Given user A continue editing
And user A copies unsaved edits
And clicks view tab (also some other tabs) – user has now open same revision R!
And clicks edit for thread T
And paste unsaved edits

When user clicks save

Then user A gets no edit conflict – there should be an edit conflict here!

I believe the error is an failure to recognize that this is still an edit conflict.