I upgraded to Premiere v15.4 today and discovered a problem editing captions. When I merge two captions together it reverts back to the default text formatting. Also any edit I make to the video track takes about 15-20 seconds to complete before I can continue playing or editing.
Issue one: style reverting to default when merging captions.
Are you applying a Caption "Track Style"? And this is being removed? Or are these limited style edits (one word italicized, or one caption with a different color font)? Such edits are always changed by applying a track style. It may simply be "as designed" that a merge of 2 captions would go to the track style or to the default.
I would file a feature request/bug report on this. It doesn't matter whether it is as designed or not; if I edit caption text to all italics, I DO NOT want it to be overridden by applying a style or by merging etc.
Issue two: video track actions slow when captions are present. There are a variety of reports of such problems. I'd file a bug report if I were you, but I have not seen this myself.