Same problem, I took hours to identity this bug, apparently, GetInstanceTransformsOnPart returns transforms started from ALL points regardless “part” index, as a workaround I have to add a “Sort” to have instanced point attributes sorted before other heightfield layers primitives.
I tested on Houdini 17.5.173, 18.0.287, all no luck. BTW, where can I report a bug to SideFX?