You could check with a stock airplane/GPS to see if you have the same issue with this approach.
The GTN750 is not managing that by itself. This is done in the AP manager (Asobo or WT one if you are using the WTT mode).
I have tested your FP with the Milviz + GTN750 v 2.1.38 + WTT mode 1.0.7.
KSDM/KCRQ Approach RNAV 24 Z transition ICUGA.
After ICUAGA, I started to descend and activated the APR mode on autopilot.
I was at 2400 feet about 1nm before JUBIM.
The aircraft made a perfect RNAV descent then.
So no issue on my side. Worked like a charm.
