@lattera where do you get to with a -v?
There is a bit of an MTU smell here, especially as you seem to be crossing a tun interface but then your client is sending 1460 MSS. But, then again, the far end is advertising a smaller 1400 MSS.
The far end never ACKs your larger 1388-length segment, though: it only ever ACKs up to seq 23 (relative).