Hello. Sorry I don't have screen shots or save game files as my reporting of this bug is a little bit of an afterthought. I can't even remember which culture I'd picked that has the district to be honest, but the problem seemed pretty easy to diagnose. Most specialty districts you can build once per territory, however with the floating market I was only able to place one per city, regardless of how many territories had a tile that met the requirements for the district to be placed.

That's it!  Really enjoying the game so far.