Has something changed in createSession for SDK > 4.0.7?

Specifically, I've had my Venu 2 update recently, and since about 8 days ago (maybe the 15th May?) my previously-working app can no longer create an ActivityRecording session.

The code itself is pretty simple:

actName = "Wading"
actSport = ActivityRecording.SPORT_SWIMMING
actSubSport = ActivityRecording.SUB_SPORT_LAP_SWIMMING
lapLength = 50
_Session = ActivityRecording.createSession({
  :name=>actName,
  :sport=>actSport,
  :subSport=>actSubSport,
  :poolLength=>lapLength
});

When I call _Session.start() I get a return code of false - there's no error handling I can see for this. And it doesn't seem to be returning a null either.

Perhaps coincidentally, perhaps not - the same device is no longer recording sleep cycles, having stopped on 16 May.

Further anecdotes/data would be appreciated.