When the brush stroke direction is inverted the brush picks color from one side and transports to the other. It seems to happen only with angle expression enabled (any mode), I had the same problem with artrage.
artists oils code stems from the mixer palette brush written by artrage guys - idt corel will be/are capable of doing anything to combat this issue.
I think that is normal/expected and is a feature, not a bug...check user manual
No one paints inverting the brush with each stroke, the dirty side should be the one that touches the paint.