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