I searched for this issue and find another thread on here with the same problem as me, but it wasn't resolved because OP never followed up with the replies.

Bug Report

When using optifine F5 with the pack, I'll get a bug where opening inventory or pausing the game causes the items in my hotbar to appear "over" the new UI being used.

Possible Solution

Clearly solution is to remove Optifine, which I did and that solved the problem. From there I kept downgrading the versions of Optifine I was using, all the way down to D1. F4 was the only working version for me (other than F5), all the others caused crashes on startup, but the bug wasn't fixed.

Of course I could play the pack without optifine, but it boosts my fps a bit. Using 6 GB of ram for the pack with java args for garbage collection. I have 32 GB of RAM in my system, idk if allocating even more to minecraft will solve anything.

Steps to Reproduce (for bugs)

Not sure how to reproduce, it seems to randomly happen. Though I notice that opening a crafting bench or the handheld crafting table will temporarily fix it.

Logs

• Client/Server Log:
  Log 1: https://pastebin.com/6HpAdAp7
  Log from world (had to use private bin cause it didn't have a size limit): https://privatebin.net/?247a13a52d5b5fe0#JDnUEgiQJ3koUk5r6ZL3Lj1WkeRtbj3DM2YnuPHvot65

Client Information

• Modpack Version: 3.1.2 Hotfix 1
• Java Version: 1.8.0_51 x64
• Launcher Used: Twitch
• Memory Allocated: 6144 GB
• Server/LAN/Single Player: Single player
• Optifine Installed: Yes, Optifine_1.12.2 HD_U_F5 , through D1
• Shaders Enabled: No

World Information

• Modpack Version world created in: 3.1.2 Hotfix 1

Hello, I think the solution to this issue is the Optifine performace option Fast Render. After finding this post and playing around with the settings, the bug seems to disappear when that option is checked off.