Maybe your touchscreen is not stock. Only a few ROM can adapt, unfortunately I can't remember which.
UPDATED: Just found the ROM, a user also face the same issue as you, SHIELD-BLOBS not support third party screen. https://forum.xda-developers.com/mi-pad/development/unofficial-lineageos-14-1-xiami-mipad-t3557616
Thank you for the link! I went and tried this and yes touch does work. There are others that work too but they are maximum Nougat. Sad I can't have O or P on the pad. Maybe will try building a working version by myself.