I am experiencing exactly the same issues with my PMS (latest version) on a Mac mini:
- Plex player (Apple TV, iPad and iPhone) not being able to wake the Mac mini
- Plex failing to keep the Mac mini awake while streaming content
With regard to issue #2, in another thread it was concluded that PMS is using a deprecated method to tell MacOS to stay awake: https://forums.plex.tv/t/plex-server-should-keep-mac-awake-after-wake-on-lan/60590/56