Terrain clamping is driving me crazy!

Here’s a weird problem. I’m flying two tailing dam walls for photogrammetry. One on the West wall, everything’s fine. One on the East, every flight I get a terrain clamping error. The last set of flights the East side was about 25m too low. Both sides are using DTM geotiffs from previous flight. Projected in wgs84 geo and CGVD28 HT2 v70 vcs. Even being a hybrid geoid model, it should be pretty close to mission planners EMG96. Any ideas?

Ardupilot terrain grid resolution, default is 100m

I’m thinking it shouldn’t matter as I’m not using terrain following. So I’m not uploading terrain info just waypoints in orthometric heights. I can’t imagine the difference between HT2 and EMG96 could be that far out. I’d expect that if the autopilot is using ellipsoid elevation. Oooh, maybe it’s using ellipsoid, it’s supposed to use EMG96 with the option to switch to ellipsoid. The area I’m flying from in HT2 is about -15 above ellipsoid.