Thatch blocks use a standard "cube" model, which doesn't have any interior planes, meaning that a player who enters said block will still be able to see outside of it, which seems wrong.

I have a resource pack which fixes this issue by giving thatch blocks a model akin to that of powder snow, such that it has planes on the inside as well, getting rid of this weird behaviour. As the current thatch model is code-generated, this can't be added through a simple PR with only resource files, so here's the resource pack itself - hopefully it should be easy enough to implement this model and switch up the code that generates models to point to the newly introduced template model:
thatch-interior-fix-tfc1182.zip

It also might be advisable to change thatch's culling behaviour so that thatch blocks next to each other cull each other's faces, like what happens with powder snow. I personally don't know how to do this.