No signal quality refresh starting (after NetworkManager auto-connection)

[colin.helliwell at ln-systems.com]

I've noticed a situation today where MM isn't periodically retrieving the
signal quality. 
I can't be 100% sure, but I think it's when I have NetworkManager
*automatically* bringing up a connection on reboot - sometimes it does start
up retrieval ok, but a few times I've seen the problem. It doesn't *seem* to
happen when I bring the NM connection up manually later.
Note: this is with BEARER_CONNECTION_MONITOR_TIMEOUT lengthened to 30 (see
other post today) - perhaps there's some state machine timings tripping over
each other.

When it's ok the log shows
   [src/mm-iface-modem.c:1433] __iface_modem_update_state_internal(): Modem
/org/freedesktop/ModemManager1/Modem/0: state changed (enabled ->
registered) 
   [src/mm-iface-modem.c:1357] periodic_signal_quality_check_enable():
Periodic signal quality checks enabled (interval = 3s)

Searching the log for those state changes in the failing case just has
   [1494597460.115274] [src/mm-iface-modem.c:1433]
__iface_modem_update_state_internal(): Modem: state changed (unknown ->
disabled)
   [1494597460.175753] [src/mm-iface-modem.c:1433]
__iface_modem_update_state_internal(): Modem
/org/freedesktop/ModemManager1/Modem/0: state changed (disabled -> enabling)
   [1494597464.099723] [src/mm-iface-modem.c:1433]
__iface_modem_update_state_internal(): Modem
/org/freedesktop/ModemManager1/Modem/0: state changed (enabling -> enabled)
   [1494597467.717644] [src/mm-iface-modem.c:1433]
__iface_modem_update_state_internal(): Modem
/org/freedesktop/ModemManager1/Modem/0: state changed (enabled -> searching)
   [1494597474.130612] [src/mm-iface-modem.c:1433]
__iface_modem_update_state_internal(): Modem
/org/freedesktop/ModemManager1/Modem/0: state changed (searching ->
connecting)
   [1494597474.378795] [src/mm-iface-modem.c:1433]
__iface_modem_update_state_internal(): Modem
/org/freedesktop/ModemManager1/Modem/0: state changed (connecting ->
connected)

i.e. Nowhere is it indicating to/from registered state. (ref. line 1459: "If
we go to registered state (from unregistered), setup signal quality and
access technologies periodic retrieval")

Any thoughts on where to dig into it next ?