I was getting the missing texture bug before realizing there was a resource pack. I enabled the resource pack but it didn't fix the problem and now mixed slabs look like this:

Hi, Is it in version 0.6.4?

I have the same issue. 0.6.4
The problem is an incompatibility with Sodium (with and without Indium).

@CebbyS Just wanted to make sure you were aware that it was recreated and has to do with Sodium.

Hi, @ShaneTRS!

Thanks for the report! I will try to investigate how to work around this.
Its nice to have a clue, that the issue is with Sodium

Hi, @ShaneTRS, @SandWoodJones!

I have updated the mod to version 0.7.1. This version should fix the issues with sodium, fabric and iris.
Can you please confirm that everything is working correctly?

Thanks!

I can confirm its working.