The compressor setting of a sequence should be an editing format, don’t use png as stated above, while that is not your only problem, that is a problem. You want to match the sequence settings to your footage not your subtitles.
Your subtitles are listed at 702x, instead of the video 720x. I try to never resize in FCP. I would make them 720x.
The other Michael gave you other things, did you take his advice or stop at the png setting comment?
You need to post a still frame of you video with the subtitle. In FCP- Export> Quicktime Conversion> Still> Tiff. Post that.