Server Version#: 1.32.8.7639
Player Version#: 10.6.0.5141
Playback with subtitles (SRT) and transcoding to 720p or 1080p does not work since two months. It only works with direct play. I have already cleared the cache and reset the App on phone. Tested on two Samsung S10 Devices. On Browser or Samsung TV is no problem. LAN or WAN makes no different.
I have found a Workaround. Go in the Plex Android Settings → Advanced → Subtitle → Subtitel Burn-In: Change from Automatic to Always.
That confirm that the Android Plex App have a bug with the Subtitle Automatic Burn in.
After update the Player to v. 10.11.0.183 and Server v.1.40.1.8227 dont fix the problem. I can confirm this bug on three Samsung S10 Smartphones. My workaround still working.