I'm having the same issue...although I can't specifically confirm that it is due to Mojave (it may have been an issue before the upgrade and I just didn't notice).
I have a MacBook Pro 2018 that I upgraded today to Mojave and I am using home sharing with my main iTunes library on an iMAC...it was running High Sierra. While songs that I have purchased from iTunes on my iMac show up in the iTunes library on my MBP automatically, I noticed that I can't use the import function to import songs from my iMac to my MBP. This is true for songs that I downloaded into my iTunes Library from sources other than the iTunes store...but it is also true for songs from the iTunes store. When I try to import the song I get the message: There was a problem downloading "Song X". An unknown error occurred (-50). I thought the issue might be related to the fact that I was running Mojave on my MBP and High Sierra on my iMac...so I upgraded my iMac to Mojave as well...but the problem remains.
Perhaps on a related note, there also seems to be an issue with how Recently Added view works via Home Sharing. If I am pointed in iTunes at my MBP Library in the Recently Added View and I click on the Album, I will see the song. When I use Home Sharing to look at the Library on my iMac, if I am in Recently Added and I click on the Album even ones downloaded from iTunes, the song is not listed. It is somehow .a function of when the song was entered into the iTunes Library...songs in the library for more than 3 months seem to be ok.
Any suggestions on how to fix this? I have verified that both my MBP and my iMac are on the same home Wifi network. In addition, if I use the iTunes Add to Library function and connect to the directory on my iMac where the song is located, I can import it that way...so it doesn't appear to be a networking issue