Wizard wasting spells

First combat I got into, against the bugs just inside the woods east of town, I had my PC Wizard cast Grease (or whatever it was called, name escapes me) at the northern two while my Priest, Rogue and Fighter dealt with the southern two (the Priest and BB Wizard had lost their weapons and the latter's grimoire to another bug that's mentioned elsewhere and the former was now using a Blunderbuss I found upstairs in the inn).

For some reason, my wizard ran directly away from the spiders.  I noticed this, re-ordered him to attack the southern group (now that I'd seen that the northern pair weren't attacking) and he then proceeded to repeatedly drop Grease onto the larger bug's head until he used up all of his 1st level spells (which covered a far larger area than I expected when I gave my PC a 15 in Int).  End result was Fighter and Priest spending most of the fight flat on their backsides and the Fighter knocked out with just a sliver of health left.  And this was after fighting just two bugs.

Was this just user-error somehow or was this a genuine bug?