What is it called when someone asks a question but it's not really root/true question they have?

In other words, they are asking a question which seems to them like the right question to ask ("How do I get to point B from point A?") when they are really at point A and are trying to get to point C.

They think they should just ask for directions to point B but if they asked how to get from point A to C, there may very well be a better answer thank first asking how to go A=>B (and eventually asking about B=>C).

I feel I read a 'fancy' name for this — a law, theorem, or fallacy of some sort and cannot find it again :(

Thanks in advance for you help!

In computing, this is often called an XY Problem.

The basic idea is that you've identified a problem ("X"), started trying to solve it, and then run up against a barrier ("Y"). Then you ask for help surmounting that barrier, sometimes without giving context of the original problem you were trying to solve. The result might still be helpful for you, but if your original problem analysis is flawed and your "Y" is the wrong approach, then everyone has wasted their time.

The first documentation I became aware of that described this was part of the Bash wiki, though it's also documented here on the StackExchange network. The Wikipedia entry for this phenomenon describes an origin from the 80s.

Barking up the wrong tree

is the idiomatic way of expressing the concept.

The phrase means to mistake one's object, or to pursue the wrong course to obtain it.

en.wikipedia.org/wiki/Barking-up-the-wrong-tree