Chain is slipping relative to large chainring but not the small one

Question

The crank on my 2016 Giant XTC Advanced (Shimano XT components) is slipping relative to the large chainring but not the small chainring. In searching for reasons many of the answers focus on chain skipping but do not believe this is the case.
I positioned the front wheel against a wall and then marked the chain and large chainring so I could see whether the chain was skipping relative to the large chain ring.
I applied pressure on the crank using only my hand and the crank slipped.  The marks on the large chainring and chain were still aligned and still in the same position so no chain skipping and no movement of the freehub.
I do not have the same issue when riding in the small chain ring and there is no movement between the large and small chainrings.
Any help is much appreciated.
REVISION
  Apologies again.

Nathan Knutson · Answer

From looking up the specs I presume your crank is FC-M8000-2. If you're saying that the crank arms are moving while the big ring and chain remain stationary in space, and the rear cog didn't move, the only possible explanations are that either the interface between the spider and the right crank has failed, or the metal toothy part of the composite XT big ring is spinning relative to the fancy three dimensional carbon body of it. Do more marks and repeat your test to see which it is.
If it was the spider interface, the smaller chainring puts less torque on it so that could explain why it's not slipping on the small ring. But looking at pictures of the crank, it seems like the spider and the arm are contoured together such that the arm couldn't just spin free. So I'm guessing your chainring is all whirly.
If the chain is moving, the suspect becomes freehub slippage, which is about a million times more common.

Edit in response to picture and note that the chain is new etc:
Your chainring is worn out.

Marras93 · Answer

By far you have to replace the big chainring. This is waaaaay abused right now.

Peter N · Answer

Confirmed - the cause was the worn chain ring; as you all know.  I got it installed today and the issue stopped.
Apologies again for the miss direction at the start; I'll do better next time.  Thanks to all the contributors to this thread.

Chain is slipping relative to large chainring but not the small one

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