I did notice that when I installed Squeezelite 1.8 on the 'Client' Pi that the LMS Pi would find it and list its IP address, but MAC address was blank - and while the PI name always presented as a player (post Squeezelite 1.8 install), 'Squeezlite' would sometimes appear as a player as well (with IP and MAC) and sometimes not. So some inconsistencies in that area.
Will get the Client Pi back to its native state and get the diagnostics to you.