Ricardo Ponce

Have you tried the suggested solutions from the documentation?

Correspondance between Uproperties and the attributes are done using either the uproperty's name (the name of the uproperty in the source code) or it's display name without space (the name used in the details panel).

The "Center of Mass Offset" property can for example, be modified either by using an attribute called unreal_uproperty_centerofmassoffset (using the display name) or unreal_uproperty_comnudge (using the uproperty name).

https://www.sidefx.com/docs/unreal/_attributes.html#AttributesGenericUproperty [www.sidefx.com]

I'm experiencing the same issue with Foliage Instances being invalid.
I did find a hacky workaround that might be useful for some of you (the downside is that it will ignore any offsets you might have added).
Following a suggestion from this thread (https://answers.unrealengine.com/questions/161217/when-deforming-the-terrain-foliage-wont-apply-to-i.html), you can Select All, Move them Up enough so everything is above the landscape, and press the key END on your keyboard. That will snap all instances back to the terrain and validate them in the process (most of them anyway).
I already contacted SideFX support but haven't heard back yet.
I will update this thread when I get a response.

@valdomat - Interesting approach! And good news, I got a response from support today regarding this topic:

This issue has been fixed in today's 18.5 daily build (18.5.399).
FoliageTypes can now be instanced via attribute instancers, and we now also update the foliage type/instancers properties using the unreal_uproperty attributes.
I've also added support for Interval type properties, which will let you set properties like “CullDistance” using an Int tuple 2 attribute:
(in the case of cullDistance, the attribute would be “unreal_uproperty_CullDistance”)