It’s not actually enforcing a 15 minute warm-up; rather, MedT waits for the next 5 minute-interval to prompt for a calibration, then waits 5 more minutes to internally “synchronize” your calibration with the sensor reading, then finally a third 5-minute period to display a sensor glucose reading.
Performing a calibration immediately BEFORE stopping/staring a sensor circumvents this problem and you don’t lose data – just bad sure to do this before forced via a “sensor end” message