In the Amazon FireTV Jellyfin app, videos will show playback starting at 0:00 when resuming a video that was already in progress or when skipping ahead, subtitles start over at the beginning and are thus not in sync.
The video timer gets reset to 0:00 due to skipping ahead or resuming a video in progress also skews what time the video will end and how much time is remaining in the video.
Jellyfin for Amazon Fire TV version 0.11.3
Server version 10.5.5
Subtitles typically used are srt format, but I have noticed this on sub and ass formats.
I have only noticed this issue on the Fire TV app and not the mobile android app or the web browser.