What happens is the fence gate can be placed infinitely, but they don't connect to fences or walls, make no noise when placing, and don't drop the fence when breaking them. You cannot drop the fence, only manually remove it from inventory. New fence gates do not have this problem, only ones picked back up after being placed.
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Thank you for your report!
We're actually already tracking this issue at MCPE-65522, so I will resolve and link this ticket as a duplicate.
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This has been happening to every block added in the nether update. It’s like a reverse ghost block, you can see it but the game doesn’t recognize it as being there, and your supply number will not decrease. Hopefully this is already known, but until then the fix I’ve found is to add the stack of bugged blocks you are holding to a chest that contains the same block and combine them. It seems to reset the block back to normal. When building with any new nether block, I’ve been leaving 1 in the chest in case I need to do this.