Mark’s Down On Your Syntax said:

Has there ever been any official word on whether a character in Doctor Who who meets an older version of themselves, interacts and affects the timeline remembers the encounter?

I’m thinking mainly of Missy and Masty here - are we to assume she always remembered what happened in this last episode before he regenerated into her? Or is it a case of time retconning memories only after the events happened to her?

Or are we just not supposed to ask?!

There is often a throwaway line about how the timelines are out of sync, or similar, thus having only the latest incarnation able to remember the events. This was a crucial part of Day of the Doctor.

In last night’s episode, however, that seems to be ‘mostly’ true for Missy, who does perform that trick that she and the Master have a vague recollection of, sans any real detail. She and Harold/Master did have the typical conversation about timelines being out of sync, as well.