MQTT topic received by Espruino sometimes malformed

Thanks for all the info. I don't suppose you received a FIFO_FULL message on Espruino's console or something like that? And have you got any other code running in the background?

Just a hunch, but you say pixel ring - do you mean neopixel? Please could you try commenting out the 'neopixel.write' command and see if it still happens?

And if it does, maybe try:

A15.set();
neopixel.write...
A15.reset();

I can't remember for sure but I wonder if the STM32 neopixel code disables interrupts, which would cause the lost characters. A15 is the 'clear to send' pin to the ESP8266, so my flipping it you can tell the ESP8266 to pause comms while Espruino is busy. Assuming that works I can ensure that the code is actually built in for new firmwares.

You can also use wifi.at.debug() to turn on debug info from the WiFi, which will help you see the actual traffic between the WiFi module and the STM32 - if none of the above works it's what I'd recommend as a starting point.