i dont know why, but Shaders put "missing textures"(the purple and black squares) on mixed slabs at times. Have tried multiple shaders, but sometimes restarting the whole game fixes it.

It happens on 1.16.4 as well. It only happens when enabling or disabling shaderpacks and it only lasts until I relaunch the game. After restarting, the issue doesn't reappear unless I enable/disable shaderpacks again. Swapping between shaderpacks doesn't incur any issue.

Thank you for the bug report and sorry for the delay in response. Does this happen specifically on 1.16.3 or is this a general issue?

Same issue. Only happens with horizontal mixed slabs when shaderpack is enabled

Hi there! Apologies for the delayed response, would you mind testing this in the latest version please? I believe that this should be fixed :)