Rebel,
You and your partner both played this hand well, and ultimately that's all that matters.
First, you made a thin but correct call. Whenever you don't block all loners Jack+1 and nothing else is a must call. Good job on that. When your partner takes the first trick, leading trump to the maker is the correct play.
There are certainly exceptions to the "lead trump to your P" rule, and those times occur when you have more information on your P's hand--information that suggests your P has a marginal holding.
EG: Say you're in the 2 seat and your partner picks up the

, seat 1 leads the

, you and seat 3 follow suit and your P takes it with the

, and now your P plays the

, and you end up taking that trick by trumping in with the

.
Let's say these are the cards you have left:
Now it's time to lead trump to the maker, so play the

right?? Actually no. Notice how your Partner played this hand so far. He took the first trick without burning a trump and yet played a garbage card on his next lead. This line indicates a very marginal holding, the kind've holding a trump lead could easily screw up. So in this example your best lead back is the suit that hasn't been played yet, the

.
Reading hands, spades is also the suit your partner has to be void in. He already played a club and a diamond, and he can't have the

becuz if he had that he would've lead trump on the 2nd lead. So by leading a spade you give your P an opportunity to get a cheap trick which is exactly what he may need given the weak nature of his holding.
Notice in your hand example, your hand was also weak, and a trump lead hurt it, but your P had no way of knowing your hand was weak. With no information on the nature of your partner's holding, the default play is to lead trump to the maker, so your P played it correctly.