I would be trying to check your LG app setting, my app works fine with a LG 65OLEDB8. HD-MA 7.1 channel audio Titles are an issue but don’t have to many of those.
You have left a few details out in your post, Version of PMS, app version, TV model, Ethernet or Wifi. Please reply with more detail.
I have a new LG77C9 TV with a plex 673 server. All hardwired. Playing 4k movies works great, but older one that are from the 80s just continuously buffer…frustrating!!! Any ideas?