Steath attacks before combat is completely broken, you can spam free attacks till you are detected and then they are surprised a round so another round goes to you. its dumb
I'm cheering here
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=760775#Post760775 for that what cases this exploit to be fixed.
As for OPs problem: that's seems perfectly D&Dish to me to fail at stealthy assassination. At TT a DM would explain that with some new factor involvement (the NPC moved a bit, you tripped a bit...). The only difference from TT that here goblins are not just your "pesky goblins" with just 7hp, so it's much more likely that you'll need more than one hit.
