As your “Video Preview window displays a split-screen preview (so you can monitor frames on both sides of the edit point)” in addition to the A/B roll, you probably accidentally pressed 5 on the numeric keyboard (or double-clicked the event edge) and got into Expanded edit mode.
From online Help: “In expanded edit mode, a red bracket is displayed to indicate which event edge will be trimmed, and the track is displayed in an A/B roll.”
To exit Expanded edit mode, do one of the following: Choose “Edit” > “Select” > “Exit Expanded Edit Mode” or Press 5 on the numeric keypad or Double-click the event edge or Press Esc on your keyboard. To avoid it, do not press 5 on the numeric keypad (or double-click the event edge)…
Lots of threads on this since it was added in VP 12.