Are the L and R edges of each video frame involved in the transition perfectly aligned with the true frame edges as per the timeline which are identifiable if you zoom the timeline fully in to maximum. All video event L & R edges must align exactly with the vertical frame lines showing on the timeline. If they don't, weird results can sometimes result.
In Vegas Pro's internal preferences (access differs depending on VP version), there is a selection to get failures in exact quantize positioning to show as a red L and/or R edge on the video or audio event on the timeline.
It's never been an issue before with alignment. (Many years) Actually... I turned off GPU acceleration and it went away. Not sure why it was on. Very strange.