Captions bug: "Synchronize" makes it overwrite itself
How to reproduce: Import any SRT file, and place it above a video file somewhere in the timeline that it should sync with. If you select all the individual text blocks using the track select foward tool, then shift+click to select the video and choose the "synchronize" option, the captions all jumble together and most are significantly shortened, and are out of sync.
It seems like Premiere is having a tough time choosing which text blocks to move in what order, and therefore, some are getting clipped or deleted entirely, because when two text blocks overlap, the duration is trimmed.
A current viable workaround is to move all the text blocks to a place on the timeline either fully before or fully after the video clip you're trying to sync to, that way no single text block's post-"synchronize" destination on the timeline will be a place where a text block currently is before the move.