After reading a few posts about problems in SU4, I’ve come to the conclusion that the installer was a bit flaky but now seems okay. Have you tried uninstalling and reinstalling? Often resolves problems.
This feels more like a combination of either user error and/or local installation issues. As suggested, it might be helpful to uninstall/reinstall the sim.
Here’s the TBM 930 handling a VNAV descent without issues: