Get random navigable point always returns the (actor) origin

This is the blueprint I use to spawn decals where NPCs can interact with them:

The problem is that the decals, spawned by my actor at (0|0|0) only spawn at (0|0|15), which is the actor origin plus the navmesh height.

I can change the actors origin as much as I want, all I get are the following results, assuming that the actor is at (x|y|z):

(0|0|15),
(x|0|15),
(0|y|15) and
(x|y|15).

If the actor is outside of the navmesh, the x and/or y value(s) will be replaced by the nearedst value that is in the navmesh.

Can anyone tell me what the cause might be or how to fix it?