Whenever I use the Item Panel, NEI does not show. I've tried hitting "o" and all the other things. As soon as I interact with the block, NEI disappears, and comes back when I exit it.

This makes using it very frustrating because you can't shift click NEI recipes, and constantly have to back out of it to go to a normal crafting table.

There's a NEI autosync option so I can only assume its a bug. I am using FTB Infinity Evolved 2.1.2 with Expert mode enabled.

I can get it to show by randomly flipping between UI scale settings and fullscreen/windowed mode. Though, not always, and it's very random.

It seems related to 4K resolution and minecraft awful UI scaling. It only does it set to "Auto" but any other UI scaling on 4K is ultra tiny.

When NEI doesn't have enough room to show (as determined by the GUIs size variables) it hides itself. You need to change your GUI scale or find some other solution, there's nothing we can do from this end.