I have had the same experience.
The RNG, BRG and ALT numbers enter into the UFC OK, but are not applied to the Mark Point in the HSI, DATA, WPT page.)
It would be great to know:
A: Were offsets available with Mark Points in the real jet?
B: The feature hasn't been implement/completed yet in the DCS F/A-18C or...
C: An actual bug exists.
I was looking forward to using this feature for some more challenging target searches, whereas you have to go find a physical landmark with the TPOD, create a Mark Point, then apply offsets to find your actual target.