Same here. MQTT does not work at all after updating to 20220613-073803/v2.1.6@166b8318 firmware. Same behavior on both of my Shelly Motion devices.
MQTT was working fine with out-of-the-box firmware but I didn't make a note of the version number 
I've reset both of my devices and am using the Shelly integration in Home Assistant instead of MQTT for now.
How did you downgrade?