Supposed to replace each other now when re-cast? IE:

You roll a 3 on your Bull's Strength roll, you give it another shot and roll a 2. This causes your strength to go down by one.

As far as I knew, the opposite was true due to the coding of the spell effects. Could this be a scripting-related issue?

Yes, I think Efu changed it so you can do that.

That sucks. If you want to try again and waste a potion/wand/spell, the best one should apply. I'm pretty sure that is the canon rules. I could very easily be wrong, but I doubt it.