Strange behavior Windows 10 firewall

I have written a tcp client application which shall create a connection to a unix system. I have following strange behavior with activated firewall on windows 10:

Thats what I can see in wireshark:
My software on Win10 starts the TCP Connection request with a SYN.
the unix system sends a SYN, ACK back
My software answers with an ACK
the unix system terminates with a FIN

This repeats all 5 seconds and after 2-3 minutes suddenly the unix system doesn’t terminate the connection but accepts it with a PSH, ACK and then the connection is ok and we can exchange data

When I deactivate the firewall on windows 10 then the connection works immediately and doesn't need 2-3 minutes.

can someone help me what setting in the firewall could cause this? I always thought either it blocks or it blocks not. But this looks strange to me

It could be MTU/MSS negotiation, for some odd reason by default the windows firewall doesnt allow the necessary ICMP packets through.

You need to allow the following in and out on windows firewall.

ICMPv6
Destination Unreachable
Packet too big
Time exceeded
Parameter problem
Echo request
Type 129

Not all of those are absolutely necessary but a good idea to make diagnosing problems easier.

ICMPv4
Packet too big
Destination Unreachable
Echo request
Time exceeded
Paramater problem

What I think is happening the ICMP packets are failing, then its falling back to TCP based negotiation so things still work but are delayed.

If you want it quick and simple just allow ICMP fully in/out, you still probably wont notice any ill affects of that.