I ran into something similar on another Dahua-based PTZ. As far as I know, the return to the preset is generally controlled by the Idle Motion or Park Action settings, but when IVS auto-tracking is enabled, some firmware versions ignore any delay and go back to the preset almost immediately after tracking ends.
I'd check if your firmware has a park time, idle timeout, or return delay option under PTZ settings, but if you don't see one, it may simply not be supported on that model/firmware.
If anyone has found a way around it, I'd be interested too. A 10–30 second delay before returning to the home preset would make auto-tracking much more useful, especially if the target briefly stops or moves out of view and then reappears.