Intensity Minutes not Recording

For the past 2 weeks or so my F6 Sapphire has stopped recording Intensity Minutes. I've altered the setting in GC Desktop to record via my HR zones rather than the default Automatic setting but still nothing

Just been for a 30 minute run with TickrX paired to the F6 and everything (HR, distance, respiration, pace, elevation etc) recorded fine ... except no Intensity Minutes

In the grand scheme of things, Intensity Minutes really doesn't matter, but it just frustrates the hell out of me that yet another 'feature' of the F6 is broken/doesn't work etc

Running latest V6.3, iOS and GCM are all latest versions

Any suggestions welcome