On MPLS interfaces, MTU is by default 1508.
The question is as follows.
If you pass the VLAN through the physical link and raise MPLS on it, then problems begin with the passage of packets more than 1496 bytes. If you raise the MTU on the physical card to 1504, then 1500 bytes packets begin to pass through the VLAN interface. But in this case, as I noticed, one of the processors on the server is loaded. And the traffic to the client does not go as it should. The speed is low.
So I thought that, in fact, by manually raising the MTU, the OS cannot actually raise the card L2MTU, but emulates a large package programmatically.