Foliage + Landscape Splines do not properly inherit/update their collision settings from the static mesh asset. The settings get independently serialized into the level/landscape, and the “Use Static Mesh Collision Settings” doesn’t function properly at all for splines.
If you change static mesh collision after placing foliage, existing foliage will not be updated to match even when it has the “Default” collision name set. There are some ancient “TODO”'s surrounding that code in engine, but it’s more complex to fix than it seems.
I’ve “fixed” this in the past by making a tool which iterates over all foliage + splines in a level before save/cook, and updates their collision settings to match the static mesh asset they’re using, and saves the level. It’s not clean, but sadly the only way I could workaround it.