I downloaded quite a few mods and also play with nh shader and the problem is that only textures are loaded as far as my field of vision was when loading the world and when I go to places where there are no textures (I can walk in there) so to speak like x ray so can see mobs and hollows) then I change my range of vision and then load the textures
You can press F3 and A or place a block in the unloaded chunk and dismantle it.
I don't care that I have to press it all the time
I got used to spamming F3+ A and F3+ D