are they getting “burned in” during playback? Because if they do, that means the server is involved, whereas if they are Direct Played, only the client is relevant. Check the server Dashboard during playback to see details https://support.plex.tv/articles/200871837-status-and-dashboard/
Are you watching on the screen of the mobile device?
Or are you “casting” to a device with larger screen?
When setting ‘Burn: Always’ in AOS, the server burns subtitles, but the subtitles seem too small in AOS APP.
When setting ‘Burn: Never’ in AOS, the server is transcoding,
Even if it is transcoding on the server, it looks normal on the Web Agent, but it looks too small on the AOS APP.
Looking at the result 2, I think it’s an AOS APP problem.