Happens while exploring, seemingly based on render distance. Reloading shaders and texture packs and increasing or decreasing render distance in-game does NOT fix it, but a save and quit then reload the world does fix it.

Mob textures, including the player's textures, also do not appear, but particle effects like torches do. Mobs do still cast a shadow, but blocks will not.

could maybe be a bug with radium or starlight, i also had some situations with invisible terrain. but it was nothing lag related and was fixed when i restarted my world. the creator radium is very inactive sadly..

Seems to be an interaction between Radium Reforged and Starlight, as the issue didn't come up again with one or the other disabled (but the pack took a massive performance hit, as expected).