I am having the same issue as described here. The fix from the device-tree-overlay you mentioned doesn’t seem to help though.
I am also using the Verdin iMX8M-Plus and starting the M7 from U-Boot using the binary file. I am trying to get this demo to work Cortex-M RPMsg Guide | Toradex Developer Center
But the M7 only boots with
RPMSG String Echo FreeRTOS RTOS API Demo...
I can only get it to work when using the old version of the overlay and Torizon OS 6.8.1.