It is not the furnishing per se, the issue occurs if the skill lands way too high above the actual (i.e. designed by ZOS) surface marked as "ground". All furnishings with collision can create an impression that they are marked as "ground" while they are not. Experiment with the elevation of this section and try to bring it closer to the actual surface. It all depends on your design, though.

Unless there is a Nav Mesh defined on a surface, the game can not treat it like the ground. Which means no Ground Based spells or targeting as you have painfully discovered. It surprises me we can even walk on the platforms we do place in our houses.

This can also mess up spells when you place a raised platform on the ground, like to make a stage. So, keep that in mind too.

This is something I completely agree they need to fix! It has bothered me for AGES. It's impossible to build something in the air for example and don't be able to place ground abilities. I know plenty of people who would wanna build a dueling arena with flying platforms where you would fall to your death if you stepped outside the platform. But it's kind of stupid if you can't use your abilities properly. Myself, I wanted to create a platform in the water of Earthtear Cavern where I would place my dummy and do my parse practice, but nope, not allowed! The ground abilities go through the platform + water and hits the seabed if the water is shallow enough. And if the water is deeper, it won't be placed at all, and will cancel the animation you're trying to do. UGH. Frustrating to say the least.