Is it normal for a 1" Lawn Sprinkler repair to leak only under pressure?
The pressure in the tubing will be lower if there is an open-end or slightly-open-end.
If the supplying valve is a normal tap (i.e. not a ball valve) that pressure is even lower, because usually the center hole of the tap is smaller than the outgoing tubing.

When you close the hole, the pressure increases measurably, because the outflow that mitigated the pressure has stopped and now the full main pressure can be applied to the tube's walls and couplings.

Now if the couplings or tube has a tiny hole, that can mean it leaks only when the end of the tube is closed.

Imagine having a 0.1" (2.54mm) hole and a chunk of clay. If you gently push the clay against the hole, nothing goes through. If you put your weight on it, you will get the clay through quite quickly.
In very small openings the water can act exactly as the clay, too low pressure and it will not want to go through the hole, but rather out the sprinkler. Too much pressure and it will squirt out, because it has nowhere else to go.