This happens to me fairly often while public speaking and (much less often) low-stress social settings. Not a complete mind blank, I do not lose awareness of my surroundings and so on, just completely lose my train of thought.

It was enough of a concern that I started keeping a diary of the circumstances. It nearly always boiled down to mind-wandering and its relatives: either focusing on a point I wanted to make down the line and not paying close enough attention to what I was saying at the moment (especially in public speaking), or thinking about something else during a social conversation. Sort of, operating on conversational auto-pilot, which seems to break down as soon as anything goes amiss (I make a spontaneous speech error & need to correct it but then suddenly am unsure of what I was saying; or someone makes an unexpected point and I wasn't paying close enough attention).

In public speaking I reduced this by switching from a highly rehearsed literal approach (writing up a script and then close-to-memorizing it), to something more based on a narrative arc, as in this post on public speaking ... I still get tongue-tied occasionally but not nearly as often, since I am only trying to get to each important point in the narrative but not trying to keep to a specific script.

In social conversation, my diary notes suggested this happened mostly during conversations that I wasn't so engaged with - not really such a concern except for social reasons it's not a great thing to zone out when someone else is talking to you.

So my feeling from an n=1 diary study (confounded with practice effects in public speaking), is that this is not a working memory problem for me, but more about distraction and focus.