The Arduino IDE uses the avr gcc compiler.
That compiler has a set of default libraries included.
But ..... not the floating point library for sprintf.
So every float results into a question mark.
If you use the avr gcc compiler (without the Arduino IDE) it is possible to link those libraries and have a sprintf working with floats. But the code size increases. I don't know how to do that with the Arduino, but it is possible.
Many of us would like the Arduino to include that library by default. But that is not yet the case.
I think "dtostrf()" is the workaround.