Looks like you had basically no fillet there, so very little contact area - even with perfect braze penetration, a butt joint isn't strong enough for joining tubes.

It sounds as if you didn't have enough heat and heat control there - the brass shouldn't be molten before it hits the steel, the steel should be hot enough to melt the brass on contact. I've never tried mapp gas, but I'm not sure you can get the kind of high temperature control that way to get good fillets.