With uPnP the port mapping is refreshed periodically and if the attempt to do it or communicate with the router control interface fails then Plex Media Server would switch to a different port
For example at 01:34:32 port 23873 was successfully mapped
Jul 20, 2018 01:34:32.826 [0x7f5d6efff700] DEBUG - NAT: UPnP, mapped port 23873 to 192.168.1.250:32400.
An hour later the attempt to refresh the mapping failed
Jul 20, 2018 02:34:25.857 [0x7f5d693fc700] DEBUG - NAT: UPnP, attempting port mapping.
Jul 20, 2018 02:34:28.869 [0x7f5d693fc700] DEBUG - NAT: UPnP, found device <http://192.168.1.253:49062/rootDesc.xml> with private address <192.168.1.250>
Jul 20, 2018 02:34:28.881 [0x7f5d693fc700] DEBUG - NAT: UPnP, usable device <http://192.168.1.253:49062/rootDesc.xml> with private address <192.168.1.250>.
Jul 20, 2018 02:34:33.888 [0x7f5d693fc700] WARN - NAT: UPnP, error mapping port 23873, error: Unknown Error, controlURL: http://192.168.1.253:49062/ctl/IPConn.
Jul 20, 2018 02:34:33.888 [0x7f5d693fc700] DEBUG - NAT: PMP, attempting mapping.
Jul 20, 2018 02:34:34.165 [0x7f5d62000700] DEBUG - NAT: UPnP, not an IGD: <http://192.168.1.253:49062/rootDesc.xml>.
Jul 20, 2018 02:34:34.165 [0x7f5d62000700] DEBUG - NAT: UPnP, getPublicIP didn't find usable IGD.
Jul 20, 2018 02:34:37.640 [0x7f5d693fc700] WARN - NAT: PMP, got an error: Network Error.
Jul 20, 2018 02:34:37.641 [0x7f5d693fc700] DEBUG - NAT: UPnP, attempting port mapping.
Jul 20, 2018 02:34:37.650 [0x7f5d693fc700] DEBUG - NAT: UPnP, usable device <http://192.168.1.253:49062/rootDesc.xml> with private address <192.168.1.250>.
Jul 20, 2018 02:34:37.918 [0x7f5d62000700] DEBUG - MyPlex: Last published value didn't change, we're done.
Jul 20, 2018 02:34:42.652 [0x7f5d693fc700] WARN - NAT: UPnP, error mapping port 23873, error: Unknown Error, controlURL: http://192.168.1.253:49062/ctl/IPConn.
Jul 20, 2018 02:34:42.652 [0x7f5d693fc700] DEBUG - PublicAddressManager: Mapping failed.
And the check an hour later - it succeeded using a different port
Jul 20, 2018 03:34:25.857 [0x7f5d68bfb700] DEBUG - NAT: UPnP, attempting port mapping.
Jul 20, 2018 03:34:33.888 [0x7f5d68bfb700] WARN - NAT: UPnP, error mapping port 14466, error: Unknown Error, controlURL: http://192.168.1.253:49062/ctl/IPConn.
Jul 20, 2018 03:34:33.888 [0x7f5d68bfb700] DEBUG - NAT: PMP, attempting mapping.
Jul 20, 2018 03:34:34.639 [0x7f5d68bfb700] DEBUG - NAT: UPnP, attempting port mapping.
Jul 20, 2018 03:34:35.076 [0x7f5d54bff700] DEBUG - MyPlex: Published Mapping State response was 201
Jul 20, 2018 03:34:38.709 [0x7f5d68bfb700] DEBUG - NAT: UPnP, mapped port 10668 to 192.168.1.250:32400.
Jul 20, 2018 03:34:38.709 [0x7f5d68bfb700] DEBUG - PublicAddressManager: Mapping succeeded for 192.168.1.250:10668.
It is more reliable to use a manually specified port and have a port forward in the router to your server local IP 192.168.1.250
You would need to make sure 192.168.1.250 is a DHCP reservation or outside dhcp range
