ATV is over Ethernet and Mac over WiFi.
I’m using a third-party plex hosting platform, and I’ve never had such issues with Nvidia Shield. It has to be something to do with Infuse on the ATV.
Also, I forgot to mention, a few days ago I had a similar issue, and the only way I was able to solve it was by uninstalling and reinstalling Infuse. I thought at the time that it had something to do with syncing with the Plex server because the refresh/sync icon (or whatever the name of this icon is, the second one in the top right corner, next to the settings icon) was in the loading state all the time. Likewise, Infuse on the MacBook worked without any problem. However, right now the syncing isn’t always on so it has to be something else.
And, as it would seem, it isn’t happening every time, but lately, it’s happening more often than not.