Are you able to reproduce this consistently?
If so, please provide more detailed instructions to us for use in diagnosing the issue further.
In an attempt to quickly reproduce this issue based on your description, we've made a simple HDA that instances objects along an input curve geo.
- In UE5, we created an Actor Blueprint with a child Spline, instanced that into our level alongside the test HDA and then sketched a few control points into the spline.
- On the HDA, we set the input type to World Outliner Input, then used the selector tool to pick the BP containing the SplineComponent and the HDA works and cooks just fine.
Also, we noted your use of 19.0.531 and wanted to mention that there are new production builds available as well, in case you'd like to try to reproduce this using latest: