Certainly an interesting issue! I haven’t seen something like that before. But it is fitting for the season 🙂
First up, it seems like your axes board doesn’t do anything? When you move a slider or joystick, is there any change on the OLED screen? Check the pinheader soldering, and the soldering and orientation of the ADC chip on the axes board. Check the voltage reference and resistor there too.
In your photo above, there is no cable connecting the neo_85 board to axes and there are no LEDs on the axes MEC switches. Can you check if the cable was connected? If so, what happens when you push the MEC switches? The board at least seems to receive +5V power as the slider LEDs are on.
For the random ePixel LEDs, it looks to me like there is some data corruption/bit shift somewhere. It would be easier if you could light up the whole array with the sliders as you would quickly spot any bad LEDs. It could just be poor cabling on the 8-pin headers or maybe a wrong THT resistor value? What happens if you plug the J44 cable from the Core into the third neo_68 board? With the J89 cable still plugged into the first board as normal, the switches on that should effectively control the LEDs on the third board.
The Matias keys don’t control the ePixels directly, instead they send a control signal to the MCU. From your video, it doesn’t look like the key-scanning is problematic. If there was a problem with the shift registers, normally what you’d see here (as noted in the troubleshooting) is that a single key press would light up a whole column of four/five LEDs on one neo board, or a whole row, or a whole column or row would be missing. You would have to do a systematic test of all keys, but I still think the issue is related to the LEDs and not the shift registers as it looks like you can address each row just fine (only one column of the third neo_68 board was tested in the video). But if you suspect a faulty 595 chip, as long as it is 595D, that is okay. The T and R refer to the IC packaging, so in a tube or on a reel.
Viele Grüße nach Aachen,