I think everyone has these issues, OSX is just terrible at file sharing. When it works it’s great, but then it doesn’t. Even if you make an IP connection, rather than the easy selection, it still doesn’t see other computers sometimes. Mixed OS, same OS never seems to matter. If you try to connect from the older machine, good luck. Funnily enough I never have trouble connecting to networked Windows machines. Seriously Apple?
I cannot count how many times it won’t un-mount a network drive, preventing sleep/shutdown. It’s been a long time, but IIRC file sharing was more reliable in Mac Classic OSs’, kind of sad really. Even when we are using Apple routers it still happens, and you’d think that would be one of the kinks in the chain.