When equipping gem armor, the protection value is being doubled. You can see the proper value being applied to the armor bar for about a tick before it jumps up to the inflated value. Not sure if this is just a visual thing, but regular-tier armor seems to provide way more protection than it should. Haven't even tried super-tier. 1.11.2 and 1.12 are known to be affected. 1.10.2 untested.

Haven't encountered this recently... maybe it was a Forge bug?
(edit) No, just happened again in 1.12.