lwm2m_engine notifications causing modem send to lock up

[HFled]

We're developing a product using NB-IoT, lwm2m and the nordic nrf9160 mcu.

What we're observing ist, that using the latest nordic connect sdk, multiple lwm2m ressource notifications cause the modem socket library to indefinitely block on send() calls.

Steps to reproduce:

1. download and extract attached tarball
2. west init -l app
3. west update
4. west build -t menuconfig -b nrf9160dk_nrf9160ns app
5. set CONFIG_SERVER_URL to a valid lwm2m server (server must observe reported ressources)
6. west build && west flash

The fw will create a lwm2m connection, report an object with 26 ressources and notify them every 5s.

This stops after 1-2 times. The issue is the send() call in lwm2m_engine.c:1012, which after a few notificatios will block indefinely.

In the tarball you will also find the uart log output (screenlog.0) and a modem trace (trace-2021-04-12T10-44-32.273Z.bin) of the issue.
6443.lwm2m_modem_lockup.tar.gz

[HFled]

The likelihood of triggering this seems to be dependant on the quality of the connection. When the connection is bad (rsrp -115db) the failure happens almost immediately.