I can confirm that this is related to the voltage of the GPIO lines… I have tried using a pair of level shifters I had on hand, with a setup looking like this:
And for some reason nothing was displaying anymore on my panel, even though I tested the functionality of the shifters (VCC A to 3V3 pin, VCC B to 5V pin, A pins to ESP and B pins to panel, common ground) with a simple GPIO sketch setting pins low and high and measuring with a voltmeter, and indeed 5V was being output (or not) at the expected times.
Anyway, maybe the shifters are too slow to drive the panel or whatever. I have gotten everything to kinda work by lowering the power supply voltage to 3.8-4V. Of course the LEDs are dimmer, but now they are perfectly controllable. I have also tested with multiple libraries and methods of driving these and it’s the same result.
This being said, do you know any good / known working level shifters with enough channels? I cannot currently go for a ready-made adapter board because I have different varieties of the ESP32 on hand and their pin distances and orders to not match. So I will have to do some kind of PCB with level shifters in which I plug the ESP pins with jumpers and from there to the panel via the connector. At least for testing/validation purposes.
Thanks for your help!