Arduino_LED_Matrix should on() only turn on LED and off all the others?

[Grumpy_Mike]

@Maaatth

I forgot to specify it in the initial post (I edited it). It's indeed the built in matrix of the Uno R4 WiFi.

NEVER add information to the first post it spoils the narrative of the thread.

What you need to do is to look at this thread where all of this was thrashed out:-
LED matrix on pin blink