Not a pro here, but I’ll venture to say it’s not normal. Some causes might be spot-setting, not using the right trowel, wrongly mixed thinset? Hopefully a pro here will weight in with what the fix is for this.

It looks like they used a small v-notch trowel, which you can see on the left side there doesn't appear to be any adhesion. ANSI A108 indicates a 95% thinset coverage in wet areas. The trowel size should be selected based on this standard. Often, the tile itself will give a minimum trowel size, but in practice, it is the tile installers experienced based on the application, the tile size, the tile material, substrate, and back of the tile to name a few considerations.

This one tile falling off could simply be a matter of the setter applied a large area of thinset and it started to skim over before he got all of the tile set. Chances are you won't have any issues, but you shouldn't have a tile popping off leaving a clearly defined trowel area that the tile wasn't fully imbedded into.

Ok I really appreciate the advice. Just want to know what level of escalation I should be going with. “Fix the fallen tile” or “is the installation compromised”

Did they soak the tile before applying? How much time was the thinset open in the wall before the tile was applied? What waterproofing is under that thinset?

Bspoke is correct. In addition look at the backside of the fallen tile. Most likely it is clean. Showing little sign if any of adhesive transfer. Do point out the required 95% coverage in wet areas. I personally would use my fist and bang the walls hard and every where while the installer is there. That tile falling is not a god sign. I often see this when folks use cement board and don't sponge it first. (give it a drink) as required. The board will suck the moisture from the setting mortars resulting in your scenario. Use a screwdriver and tap around. You will hear the difference when tile is hollow (not bonded).

Which brings us to the question of waterproofing. In progress photos are the only way to help you with that topic.

Not sure why soaking tile was suggested above as we have no idea what the tile is being installed. I very highly doubt the body is that soft / porous to require soaking.

My concern would be that that one tile is a symptom of a wider failure. Seeing what the thinset looks like behind that tile, there is no guarantee that you don't have more loose tiles waiting to fall off in 2 weeks or 2 years (when you're not going to get the contractor coming back to fix it). Grouting may help keep poorly adhered tiles in place longer, but they will still ultimately fail. I'm not sure what to tell you about how far to take this, but at a minimum you should rap with your knuckles on every tile to see if you get any hollow sounds. If you do, then I'd be pressing the contractor for a complete redo of the tile work.

I agree with Kudzu. If one falls off you have a larger problem. I'd have no problem banging on the walls with my fist to see how many fall off. You should be able to beat the walls until your hands are sore with nothing falling off.

Did they use thinset or mastic?