STM32 Cellular/LWIP/PPP 100ms periodic wake after shutdown

[chrissnow]

Description of defect

During power optimisation of a design it has become clear there is an issue with clearing down a cellular connection.
If I allow the modem to get as far at attach then shutdown everything is fine but if we use the normal connect\disconnect the device will wake up every 100ms forever and I cant figure out why or what it's executing!

sleep tracing shows the following.

deepsleep locked by: [lp_ticker.c x 1] [SysTimer.cpp x 1]
deepsleep locked by: [lp_ticker.c x 1]
deepsleep locked by: [lp_ticker.c x 1]
deepsleep unlocked
deepsleep locked by: [lp_ticker.c x 1] [SysTimer.cpp x 1]
deepsleep locked by: [lp_ticker.c x 1]
deepsleep locked by: [lp_ticker.c x 1]
deepsleep unlocked
deepsleep locked by: [lp_ticker.c x 1] [SysTimer.cpp x 1]
deepsleep locked by: [lp_ticker.c x 1]
deepsleep locked by: [lp_ticker.c x 1]
deepsleep unlocked

Target(s) affected by this defect ?

Likely all combinations of targets that use PPP and Cellular.
I am using a STM32WB and a Quectel EG916 (sng the EC2x driver)

Toolchain(s) (name and version) displaying this defect ?

ARMC6

What version of Mbed-os are you using (tag or sha) ?

Master

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-cli

How is this defect reproduced ?

Use a cellular modem in an application that should deep sleep.