you redid it, so you can tell us if the plumbing is all in a thin space under a slab -- if so Tomahawk it is not vented much, and maybe we'd like to see the diagram you can draw since you or your contractor saw it all before closing the ceiling. If not, you can open your own ceiling and see your neighbor's plumbing above, and report back on what you see.
Since there are 29 floors it stands to reason there will be at least a couple other owners who know a lot about the building's plumbing, and the condo association board members will know who they are, so now you know who to talk to and how to get in touch with them.
Since your condo is the only one (or not) that has this problem, it is likely that something was changed when you redid it all. A diagram will help understand what you have or think you have. Another good reason to post this.
How do you know the floor drain P trap is full of water? Did anyone pour water down it? How do you know it has a P trap? Is it a new floor drain, converted from a toilet drain ?