When you have Skyblock mod:

• Whenever you look at the Skyblok but there are blocks in the way, the buttons will be invisible:
  
• Whenever you look directly at the Skyblok the buttons will be seen but only those ones on Skyblock:
  
• Whenever you see only Skyblock everything is right:
  

But all the buttons work even if they aren't seeable

No, the mod is called "Skyblock" it adds a block called "Skyblock" and it makes you see only sky through this(unless you give it redstone signal).

Yes, but there is no mod called "Skyblock" (at least i couldn't find it), so could you send me a link?

Which skyblock mod? Ex Nihilo?

It is called "Skyblocks" with "s" at the end. I'm sorry but I think one letter makes no change https://minecraft.curseforge.com/projects/skyblocks

Honestly i have no idea how to fix it, might be a good idea to contact the author of skyblocks as well.

Ok, I will do it then.