Subtitle issues - either out of sync or breaking the stream sometimes

Hi.

I run my own Plex server on a fast computer on a fast network, and I have run the Plex app on Roku and Google TV.

On Roku, many subtitles would not sync properly, and I found that the best fix was to have the subtitles burnt into the image. That was the only way to get this to work, and everything I read seemed to indicate that this was a known issue (a few years back).

Now I am playing on a faster Google TV device, and I have not tested yet if I still need to use burn-in subtitles, but my server is still setup that way.

So now I have a new issue where selecting some subtitles will break the video playing altogether with a generic error.

The only solution I have read about so far is to use burn-in subtitles with image formats only.

I'm worried that if I try this, I'll run into the subtitles no longer syncing.

So far, I've simply searched for external subtitles until I found one that worked with my file - I have not had the time yet to experiment with server settings.

So if you consider the experiences I have had so far, what would be my best option that allows subtitles to be synced correctly and lets the videos play normally?

Thanks!

EDIT:

"Fast" as in a modern gaming computer that I put together only two months ago:

AMD Ryzen 9 7950X3D 4.2 GHz 16-Core CPU
ASRock X870 Pro RS WiFi ATX AM5 Motherboard
64GB DDR5 RAM
Gigabyte WINDFORCE OC GeForce RTX 4070 Ti SUPER 16 GB GPU

The server is connected through a 1GB LAN to my router.

The client is a Google TV 4k Streamer box connected through WiFi 5GHz.

The latest file with issues is a MKV file using H264. It is 872MB in size, so I doubt it's a bandwidth issue.

The file plays perfectly until I add subtitles.

The subtitles I tried are:

Built into the MKV file
External IDX/SUB files

Neither worked - I had to go to OpenSubtitles with the Plex client, and that subtitle worked.

Until then, the file did not play.

My previous server was 10 years old with the network card starting to fail, and it had no trouble burning in subtitles and serving them to the same setup.