Both server or client is able to send the TCP segment along with FIN flag that is set to 1. Whenever the receiving end reverts it back by Acknowledging the FIN, that particular direction of the TCP communication gets closed and then the connection gets released. Working with TCP Protocol

TCP Keep-Alive Set when the segment size is zero or one, the current sequence number is one byte less than the next expected sequence number, and any of SYN, FIN, or RST are set. Explanation of the Three-Way Handshake via TCP/IP Feb 12, 2010 What is TCP Protocol? | How TCP Protocol Works?

The TCP ACKed lost segment means that Wireshark missed at least one packet in the other direction. The receiving IP stack would not have ack'd unless it received it. So I'd check my placement of Wireshark and make sure that it is in the path for both directions. If so, is it on the one of the endpoints?

If a TCP segment is too large, it will lead to an IP datagram is too large to be sent without fragmentation. Fragmentation reduces efficiency and increases the chances of part of a TCP segment being lost, resulting in the entire segment needing to be retransmitted. TCP Default Maximum Segment Size How does a TCP Reset Attack work? | Robert Heaton Apr 27, 2020

Transfer Control Protocol, 3-way handshake, TCP sliding window

The receiver may re-ACK previous data when an out-of-order segment has been received. This hints to the sender that some but not all of the pending packets have been lost and so the sender may choose to retransmit only a subset of what is in its outgoing window (and thus result in a smaller network packet and less overall bandwidth use) rather than sending a bigger packet that includes data Recommended TCP/IP settings for WAN links with a MTU size Apr 17, 2018