This more a bug as a feature after forced subtitles worked once in the past.
At least with the old player forced embedded subtitles worked on my apple tv.
But in the meantime this broken, too 
I really don‘t understand why it‘s so hard to support the forced subtitles flag in mp4 files… this is since ever troubling with plex
What exactly isn’t working for you?
I have a bunch of mp4 files with embedded subtitles (full and forced ones) which are recognized and showing alright.
As it’s not actually a feature suggestion (as you pointed out yourself)… let’s move this off the feature suggestion category 
the problem is that forced subtitles in mp4 files are not shown on any device, not only the AppleTv.
There are a plenty of threads in the web that this issue is ignored by Plex devs since… forever
examples Reddit - Dive into anything
so I think it‘s still an open request
The mp4 container is quite powerful when it comes to associating subtitles to specific audio tracks and defining what forced subtitles belong to a full/regular subtitle (or if a subtitle contains forced elements at all).
I’ve noticed that Plex seems to recognize stand-alone forced subtitles inside an mp4 container and auto-selects them – that’s not the case with linked/associated subtitles inside the same file.
I’ve also noticed that while the subtitles show up, they’re not tagged as forced inside Plex.
Mind sharing some details about your files that aren’t working properly?
But it IS a feature request. It’s not that the subtitles don’t show up. It’s that Plex ignores the Forced flag when it’s an MP4. For as long as I’ve known, Plex has never recognized when one of the embedded subtitles has a Forced flag, except with MKV files.
Calling it a “bug” implies that Plex is meant to recognize Forced subtitles in MP4 files, but I don’t think that’s the case. I think they just never coded Plex to do that.
Still curious how you tagged/flagged those subtitles as forced, given the concept of forced subtitles in mp4 is trickier than that of mkv. Personally I like how you can assign subtitles to audio tracks and configure which subtitle is a forced version of another subtitle (or if a track contains some/only/no forced samples)…
I take it that makes it more ambiguous and there’s probably muxers/editors that’ll support that to different degrees.
I don’t know about what’s trickier or not, but it’s doable. I use an app on Mac called Subler to set the Forced flag. I don’t know what other apps will do it, because I was already using Subler for other stuff when I started getting into Forced subtitles, and never had a need to find another app for that.
It’s been a few years, but the last time I tested it, VLC correctly turned the Forced subtitles on automatically, if the flag was set. And the Forced subs weren’t even the first English subs in the file. When I renamed the file (so VLC didn’t recognize it as the same) and switched the Forced flag back off, it no longer automatically turned the subtitles on.
I read a comment elsewhere (a different thread on the forums I think) where somebody said that Quicktime recognized Forced subtitle flags in MP4’s as well. But I haven’t tested that.
Hence my question
In Subler you can apply a number of subtitle related configurations to tag something as a forced subtitle (or specific forced subtitle related functionalities).
One option is to simply specify that a subtitle track has forced samples (setting Forced to All Samples Are Forced). Alternatively you can specify/reference the Forced track inside another (regular) subtitle (e.g. if the forced aspects aren’t included in the same file or use different time codes).
Bonus question: did you assign your subtitle from the related audio track or just leave them “orphaned” in the container (see Subtitles option on the audio track)?
https://bitbucket.org/galad87/subler/wiki/Subtitles%20Guide
Edit: just trying to understand if different configurations produce different outcomes; from what I could find out, you were right and it’s simply not supported at this point – still leaves you with the workaround of exporting the subtitle track and name it with a .forced suffix; Plex will then properly pick the subtitle up and recognize it as forced.
This issue is haunting me forever.
It worked pretty long on the AppleTV, while Plex did used the original AppleTV player.
Because the original AppleTV player is handing the forced subtitles as expected.
However, the new player from Plex is broken.
But for a long time, when you choosed on the AppleTV the old player, it still worked. But since a year or something, they updated also on the AppleTV the “old player” option to the new player and since them forced subtitles are also broken with the old player.
The forced subtitles never worked on the web or iPad.
Sadly, Infuse has the same issue with forced subtitles in mp4 files. They are totally aware of this bug and have since years on their backlog, but are not fixing it.
However, I also used Subler and after it worked with the original AppleTV player and with QuickTimes it should be configured fine.
Here’s an example config of what I did:
With the implementation of multiple edition support (even if it’s not 100% what I would want), this has become my most wanted fix in Plex. I have a large media library that is 99% MP4 and not being able to set forced subtitles automatically is annoying as all get out.
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