I find that it is a greathelp the fact that spells are redirected or cancelle when their target are no longuer there, but I don't see why it also applies to beneficial spells.

Example:

One last foe and one party menber paralyzed.

I command the foe to be attacked and the priest to dialko the paralyzed menber.

The foe is killed.

The dialko spell is cancelled (there are no longuer enemies).

The xp (and loot?) is distributed among the five able party menbers.

I don't know if there's any option to disable it (no offline installer for new version), but I remember this being a problem in Wizardry Gaiden 4, which has cloud spells.

WG3: If you're hit by a cloud spell, you can prevent taking damage in the last combat round by casting the game's Purify Air counterpart.

WG4: That no longer works. If you kill the last enemy before the Purify Air counterpart goes off, it won't be cast, and your party will take damage from cloud spells at the end of the round.

In Wizardry 1, there are no cloud spells, but there is poison, and I believe poison will take off 10% of the HP of someone with at least 50 HP remaining each round.