In 1.16.5 with Optifine installed, I get a pretty nasty render bug with Render Regions enabled

Update your graphic drivers

Is it just secret rooms and optifine?

Graphics driver is up to date. I have exactly 100 mods in my modpack and I've noticed that with certain mods and with Render Regions enabled on Optifine, it causes the same issue. It's weird because if I place one Ghost block, nothing happens, but if I place another above that one then everything gets weird. Also, with different blocks come different graphical bugs. When I placed a ghost grass on top of another ghost grass, the water and things disappeared like the image above...but when I placed a ghost wooden plank on top of another, many other blocks started turning invisible (like wood and stuff). It's really weird, if you want, I can send you the zip file to install my modpack on curseforge and you can check it out yourself?

I forgot to mention that, when I said "certain mods" I meant ones like your mod, chisels n' bits, etc. They all cause the same weird issue

Does this happen with only optifine and secretrooms?