Do you have a timeframe in mind when HE will support segmentation of foliage actors based on World Partition grids? Currently in UE5, manually painted foliage automatically gets segmented into multiple Foliage Actors to make them stream, while HDA foliage doesn't know about WP grids yet, of course.

Might rebuilding world partition foliage help with this? documentation [docs.unrealengine.com]
Sorry i haven't tested if it does help, but it might be worth giving it a shot.