Having issues with BLF and devicesandusers mode

I’m setting up a clean piaf install to replace an aging one.
In the process i’m moving to devicesandusers mode
It all seems to work fairly well and the provisioning is awesome, several attaboys to the freepbx crew for that.

The problem at hand is this.
If i login at an extension a blf configured to look at my login stays red (unavailable/inuse) (*11)
If i then reboot the phone it turns green and seems to work as one would expect.
if i then logout (*12) it goes red (which would be expected)
then login again and it stays red.

looking at core show hints i can see that after I logout the number of “watchers” drops to 0 (only one phone is configured at this stage) from 1 after a reboot.

I’m using freepbx 2.8 and asterisk the phones are grandstream gxp2000s.

I’m wondering why the number of watchers is changing when i login and out, should it not stay the same and just change the status of the line?
Or is the grandstream not listening to some message telling it to start watching again when a login is processed? (sounds unlikely)

I wonder if its something to do with a blf subscription timeout or some such i should be playing with. I haven’t been giving it alot of time between a login and expecting the light to turn green.