ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ
Then complete the fast until night.
“Complete the fast” requires preserving the fast as a legally valid act throughout its prescribed duration. Avoidance of actions that contradict fasting is therefore connected with the command to complete it.
The explicit instruction is to complete the fast; consequences necessary for that completion are derived from the meaning of the act. Involuntary emission is not equivalent to deliberate prohibited consumption or intercourse during the day.
This sort of inference does not justify turning every possible precaution into a stated obligation. The legal meaning of fasting defines which acts are incompatible with it.