I’m really NOT liking what I’m seeing here. I have extreme doubts that the water has not penetrated the surface and is now behind the wall. Eventually an entire wall of tile is going to end up coming loose. I would put the likelihood of this at 90%. It won’t happen tomorrow or the next day but eventually it will.

If this is your home I would get it taken care when funds allow. I would recommend not showering in there for a few days and even put a small box tan or whatever pointing at that joint just to get some airflow going to make sure it’s to reduce any moisture that has penetrated the surface. If it’s a rental, I’d recommend using the backer rod and a silicone caulk. It’s not ideal but at least it should stop it from getting exponentially worse.

I really hate saying this because I’m not bragging but just to give a background. I’ve owned a flooring company for 34 years specializing in all sorts of tile, remodels, etc. We have done everything from residential homes, schools, hospitals, to military bases, etc.