Term for knowledge that is unintuitive but obvious in retrospect?

Once learned, some bits on knowledge are so intrinsic to one's job or skill that one cannot remember what it felt like without that knowledge (and therefore difficult to document or teach to newcomers).

What is the name for this phenomenon?

I would call it tacit or implicit knowledge, for which a good definition is

Very loosely, tacit knowledge collects all those things that we know how to do but perhaps do not know how to explain (at least symbolically).

That seems to fit OP's context very well, in that it's not easily communicated to others.

This may be called procedural knowledge:

In cognitive psychology, procedural knowledge is the knowledge exercised in the accomplishment of a task, and thus includes knowledge which, unlike declarative knowledge, cannot be easily articulated by the individual, since it is typically nonconscious (or tacit). Many times, the individual learns procedural knowledge without even being aware that they are learning (Stadler,1989).

It's also known as tacit knowledge:

Tacit knowledge (as opposed to formal or explicit knowledge) is the kind of knowledge that is difficult to transfer to another person by means of writing it down or verbalising it.

In a similar, more general, vein is situated knowledge:

Situated knowledge is knowledge specific to a particular situation.
[...]
Knowledge generated through experience is called knowledge "a posteriori", meaning afterwards.

In a less technical sense, I might simply call it wisdom, though this may not connote the full intent of your meaning. From the perspective of a novice, the knowledge may be recondite or abstruse. If you're looking for a word to describe the difficulty in learning a job or skill, you might say it has a steep learning curve.

Without sounding flippant, I would say that this "phenomenon" is learning itself (or, the learning process). I'm sure there is an academic journal out there where educators have coined a gussied up term for obstacles encountered by a mentor imparting knowledge, but I don't know of it.