Hi Brendan,
Happened to my one too, Changed the CV's on it, worked for a bit and it now has gone back to not working again.
TBH given it is 6 Wheel pick ups the Stay alive is not really needed once your track is clean.
This was the advise from a FB page:
"The CV value for 113 was 0, Strange as I had not done any programming on this at all. Anyway set it to 127 and it works again"
No one from IRM chanced to pop onto my initial query to comment either and yes these were IRM supplied Decoders so I wonder is this a fleet issue? Happened to my Deltic too.
Hope the abocve helps