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There should always be a 3-way handshake when starting a new TCP connection.

(Corner cases exist but are very rare. Ex. RFC 7413 "TCP Fast Open")

The PSH/ACK doesn't make sense either even if it uses the same ports server 80 to your 61738.

You may be facing an issue with a device doing NAT where there are not enough resources (port exhaustion) to NAT all connections.

You may also be facing an issue with a device like a load-balancer or firewall that doesn't properly or promptly clear its state table.

I suggest you check that your curl client is using a "fresh" (new) TCP source port when connecting to your web server.

Good hunting.