I've checked my Meade LPI-GM Advanced (Touptek imx178 rebadge) and see a maximum value of 16k, which matches what you're seeing except that I'm using a 14-bit sensor. I see the same thing with my 14-bit Canon T7.
It seems to me that what you're seeing is normal, or at least, not a problem. I stack with siril so unfortunately don't know if pixinsight needs additional configuration to handle this. It is odd that you don't see this with the ASCOM drivers, though.