Well, I didn't know where to put this, but it's clearly a bug.
Several Prexes decorations, have an incorrect "size" of hitbox?, if you can call it that. Which causes that they cannot be placed correctly as they are considered larger than they really are. This seems to happen with the most recent ones that have added.
I don't have them all, but it happens with the following Prex: Xaku, Gyre, Citrine, Voruna, Styanax, Deimos Kymaeros, Deimos Flagellocanth, Deimos Lobotriscid, Deimos Scissus. From what I suppose, it must happen with all the other Deimos Prex as well.