Well mine has been stuck on 100% for about a year, however the mileage has been decreasing. I filled the tank up at a Adblue pump and probably overfilled it - I got warnings for a couple of weeks afterwards! The mileage has now come down by 10000 miles (yes really! - ten thousand) and today it finally showed 95% full.
So it looks like if you overfill, it goes to something like 200% measured capacity (must be a long filler neck!) but it will decrease and start working again eventually.
I mentioned it at the dealers - Mazda are aware and will drain the tank (for a charge) to get it working again. Leave well alone if it costs is my advice!
My next plan is to wait until it shows about 50% and then put 5 litres in, then see what it goes to. That way I can be confident its working OK. If the mileage readout was to be believed, I had enough Adblue for 26000 miles when it was full!
It's poor design - either you should not be able to overfill, or if you do then after a short distance (say 500 mies) it should start to work again. This looks like there is an awful lot of the tank above the 100% full (according to the gauge) which you have to use before it starts to read again, which isn't a good plan!