Assert in subsys/net/buf.c during concurrent UART ISR and BLE communications

Question

I'm working on a 52840 board controller project that requires serial communication with the 9160 (on a nRF9160DK board) and BLE communications to an Android tablet. I have discovered when receiving serial data while transmitting gatt notifications results in an ASSERT generated at line 507 in subsys/net/buf.c. I have worked around this problem by serializing comm access on the 52840 (i.e. the 52840 only requests serial data from the 9160 when it's finished with BLE access). While this works, I would prefer to allow the 9160 to send data indepent of BLE comms on the 52840. Can you explain why I am seeing this ASSERT? Is this a known problem with a known solution? 
 I have attached my UART ISR code and screenshots of the 52840 overlay and other relevant code. Thanks in advance for your assistance.

user109868 · Answer

Hey Simon, I finally got a chance to test that config, but I still got the same error. I even tried to de-couple the UART event from the BLE transmit logic by inserting queue logic between them. The error changed from an ASSERT to the message "Unable to allocate TX context". At the point, BLE operations just seemed to hang. 
 However, in doing more research I found a related Zephyr issue (can't remember where), and it suggested switching to a thread vs a work queue. So I switched my BLE logic to a thread and Voila! Success. So this seems like an issue in the Zephyr logic somewhere. But at least I'm able to make progress again. Thanks for your help.