I fucked about with it in standard some but it's gonna be real hard to accurately tell how many of them are gonna hit on the return bounce.
Molten strike they do make an effort to move back to the original strike location, but it's...half hearted sometimes? Like they move in weird ways and often don't travel as far as I'd like them to. Maybe just running something like increased AOE in the link to get bigger ball areas is the best way to ensure the maximum of them hit after their return bounce. Interestingly if you have an additional strike the balls produced from it attempt to return to your primary strike location, which seems odd but also potentially good? But I'm not sure how you'd set up getting 2 strikes out vs a boss anymore cause I think they nerfed all the ways you could do that.
Fissure on the other hand does appear to just bounce entirely in place. They land and bounce directly up and back down and explode again. The good news though is the fissure balls (at least visually) have much larger AE than the molten strike ones, so even without them moving they should be relatively reliable in hitting twice.