Hi @gc3!
Let’s try with this set:
ytx-v2-firmware-aux-0-14-testing.bin (11.1 KB)
ytx-v2-firmware-main-0-14-testing.bin (74.5 KB)
I came up with something of a way to check how good the MAIN <-> AUX comms are working and it is by just counting how many failed events happen when MAIN tries to send a message.
It is printing this every time it happens.
It will print total accumulated number of fails, then if it failed because of timeout you will see the timeout microseconds, and then “3,X” or “4,X” where 3 or 4 are encoder switch or digital and X is the index that failed, and then you will see the read index and write index of the feedback buffer.
These prints are on lines 890-903 of feedback.ino in branch bugFixFeedbackUpdate.
I tried 2 times for 5 minutes sending 4 notes ON and 4 notes OFF every 100 milliseconds and got no failed events.
I can’t see any more stuck or no-show pixels.
Please test them when you can 
If you still see stuck pixels, we can try lowering the Serial baud of the main <-> aux link.