Delta,
They are connected, in a network, using SMB and AFP, via WiFi - as they always have been for years.
The reason I knew it was happening is because I'd be moving a folder of say a gig, and it was transferring awfully fast, so I looked, and - lo and behold - not everything was there. There is no error message.
As I said, it has to be related to Mojave, as no other changes were made to the network. That eliminates everything but Mojave, as far as I can ascertain.
Airdrop works fine except I find it cumbersome because I need to see the folders / files. As part of work, I do this all day every day.