I finally managed to solve the problem with the Witty Pi 4 L3v7 + Raspberry Zero 2 + Inky Impression 13.3"! You need to lower the SPI speed. In my case, I edited /boot/firmware/config.txt and added:
dtoverlay=spi0-0cs,clockfreq=2000000
It seems that the Zero 2 has an SPI bus that is less tolerant to impedance, and the signal degrades when passing through the Witty. If you lower the SPI frequency, it gets fixed.