Podcasts on iMac not synching with iPhone

I've upgraded from a 2009 iMac to a 2017 iMac.

I'm having issues with the Podcast app synching. Episodes from the Podcast app on the 2017 iMac never seemed to reliably synch with the Podcast app on the iPhone resulting in a mess of old episodes from the 2009 iMac and new episodes from the 2017 iMac in the iPhone Podcast app library.

I unfollowed and cleared the library on both the iPhone and iMac. I then followed three podcasts and downloaded one episode from each. Everything is ok with the iMac library, but the iPhone behaviour remains odd. The iPhone synched the first two podcasts perfectly, but when I added the third podcast the iPhone only synched that one and the first two disappeared from the iPhone library.

I've since been downloading and synching to see if there's a pattern, but all bets seem to be off regarding which podcasts sync and which don't. What's going on?