If a player or GM just wants to describe a brief scene for the sake of flavour, such as the GM throwing in a scene at the end of a session just to foreshadow events in the next, can they also Prime an Aspect in that, or can Aspects only be Primed if there are Stakes in play?

Wouldn't that make the GM get a "free" Prime ? I think that a flavour scene is a good idea to keep players interested and you could still Prime an aspect and play stakes, then end the session. I'm pretty sure that players will want to continue playing to know what comes next. A bit like "To be continued..."

Anyway, that's only my opinion.

That's what concerned me. We kind of agreed on a rule that you can only Prime or increase the Suffering on an Aspect if there are stakes involved.

The point of adding an extra scene at the end was precisely to do a "to be continued" part, to hook the players on what would happen next so they'd look forward to the next session.