Here+ base station showing wrong "Z Values"

I have been recently testing Here+ RTK(firmware version 1.30) with Pixhawk2(Standard Cube version). After the observation with 1m accuracy, I found that the X and Y coordinates of the base station were very precise. But I’m more concerned about the Z value, which is showing approximately 90m error compared to the actual value.

That is,
Actual Z value = 866m
Here+ Base station reading after observation (z value) = 777.598m

The survey was conducted in open space with no trees or any obstacles around and there was a minimum of 20 satellites connected during this observation.

Kindly help me how to overcome this problem.
Thanks in advance.

How do you know the ACTUAL Z value?
Could it be that HERE reports Z from geoid, which can be different from actually surveyed elevation?

This is related to the geoid EGM96 and the way ublox calculates MSL.
I want this working,…I wish I colud write the code to do it.