I mean, yes, that was my instinctive response based on just the title. It's always the MTU. But in this case the packet that's being lost is a pure ACK.
No. To repeat: the client is sending a pure ACK with no data. You can see that packet with a 0 byte payload length in the screenshot. That packet is also getting lost; if it weren't getting lost, we'd be receiving a SYNACK.