I believe that is normal based on a few hours of cx travel with my twin heat pump unit. I observed what you described.
My background is an HVAC design engineer, so here's the geek out reason: normal AC cooling creates a cold evaporative coil and condensation on the inside, which is connected and spiked to a condensate drain (wheel well). In heat pump operation, that indoor coil is hot and creates no condensation. However the outdoor coil becomes the evaporative coil and becomes cold, and will potentially ice up. During defrost cycle, the heat pump warms that iced up outdoor coil to remove the ice to continue heating operation. What you're seeing is the melted ice from the defrost cycle spoiling from the roof. There's usually no good practical way to capture and contain and drain pipe that defrost melt water.