When I started training my wife, she had very similar form issues on bench. And it ended up being one side was much stronger than the other. As I recall, the bar traveled up faster on her left side than on her right. I first thought that that meant her left side was stronger, but it turned out that she unconsciously shifted the weight to her right arm, which was stronger and did the lion's share of the work to get the bar up.
I convinced her to lower the weight (she could press the bar by itself fine), watched her technique carefully, had her do lots of benching at that light weight for about three weeks (benching even on lower body days), and after a deload, she started going up in weight and hasn't had a noticeable imbalance since. Something to think about.