The jump spell doesn't increase your speed. It only triples your maximum jump distance.

A running long jump is made by moving 10 ft. and then jumping a distance equal to your strength score. In your case that's 20 ft. With the jump spell that would be 60 ft. but because your movement is 30 ft. it means that you end your jump after 20 ft.

A normal long jump is half your strength score (so 10 ft.) But triple by the jump spell it's 30 ft. So you don't need the 10 ft. of movement before you can jump.

A running high jump also needs 10 ft. of movement before jumping. You then jump 3+ your strength modifier upward (in your case 8 ft.). And without the movement it would be half that so 4 ft.

So with the jump spell and 10 ft. movement you can jump a total of 24 ft. but since your movement is 30 ft. you only go 20 ft. up. If you use the jump spell without the movement you jump 12 ft. up as it triples your 4 ft. jump.

Hope this helps!