Why is this a bug? Clerical spellcasters use different names, and the game is autocorrecting your wish to the closest thing available.

Because acid ball is a valid spellbook type (in the abstract) and it should correct to rain of sorrow instead of the closest thing. I'm pretty sure I remember it doing this in previous versions, as well. Basically, it should correct to the clerical spell if you give it the non-clerical spellname, as that makes the most sense (and players aren't aware they're going to get screwed otherwise).

As you never can find a spell book of acid ball when playing a cleric the game IMHO is behaving correctly. You should learn the names of the things you can use. I also do not think that this ever has changed.

BTW, returning spell books with a completely different name probably also would confuse people. And I do not want to have to deal with bugs like "I mistyped and wished for a 'spellbook of acid bal' as a cleric and instead got a spell book of 'acid bolt'" or whatever (as typos are handled on a probabilistic base there are no guarantees on what the game might decide).

That seems to disregard the entire foundation on which one plays and wins the game - knowledge of the player, but not the PC, that is used to successfully resolve previously encountered threats and items by knowing what they are outside of the game.

I find this rejection particularly puzzling because we are talking about wishes, where like 95% of the time the PC has not encountered the item in game and has no reason to know it exists.

The spell names are listed in the manual. The player certainly has access to this information. You could even argue that the PC does as well as it is 'common knowledge' of a sort.

From the in-game perspective priests will have zero sense to wish for spellbooks of acid ball because they do not use this name (or possibly don't even know it). In any case, blame the djinni :D

That said, Thomas shouldn't have changed the engine to give scrolls of corruption removal when wished for scrolls of chaos resistance because it follows absolutely the same in-game logic as this issue. The only thing different is the old-time players' perspective (but not the new players' perspective). From another point of view, new players don't even know about the existence of these scrolls until they find it. And old players don't know this was changed and thus also have to find out the hard way.

Seriously, we need a built-in item compendium and an rfe to restrict items that can be wished-on to the ones which were found in previous games. People who oppose spoilers would surely upvote me :D