A short flight and an unplayed short flight differ
Law 12 requires the restart to reach the ten-metre line, but play continues if an opponent plays it before it gets there. The receiving team’s action can therefore matter before the ball completes its flight.
Suppose a restart is dropping eight metres from halfway. In one invented sequence nobody plays it before it lands short. In another, a receiver steps forward and catches it there. The location is similar; the intervening reception changes the law’s treatment.
Watch the receiver rather than only the distance
A commentator may say the ball looked short while play continues. That observation can be accurate about the flight without establishing an infringement after the opponent has played it. Locate the catch or touch in relation to the ten-metre line.
Wind can also make a ball cross the line and then return, so its eventual landing point is not always proof that it never reached the required distance. Follow the complete path. The key viewing distinction is between a ball failing the distance untouched, a ball played early by an opponent and a ball crossing before blowing back.
Questions answered
Must play stop whenever the ball is caught short?
No. An opponent playing it early is specifically relevant.
Does a short final landing prove it never crossed ten metres?
No. It may have crossed and then blown back.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.