Unable to Configure MQTT Client ID using Adafruit MQTT Library
The Client ID is expected to be stored in flash memory (it is read using pgm_read_byte). However, on the Arduino, unless told otherwise it first copies the string literal into RAM.  That means that the function is getting the address of the string in RAM, not in flash, and using it as if it were the address in flash, and that gets gibberish.

You need to force it to be in flash by creating a PROGMEM string:

const char clientid[] PROGMEM = "clientfoo";
Adafruit_MQTT_Client mqtt(&amp;client, "10.116.10.191", 1883, clientid);