Ok, this is a thing, but i dont feel like its a very commonly used term anymore.
A fugue as a form can be very strict with a lot of rules to follow and the expectation of employing specific contrapuntal devices. A "learned fugue" would not deviate from the rules, would not drop into homophonic sections, and would use the expected devices heavily. It would be a stand-alone piece (although they often have preludes).
By contrast, a larger work might have a fugal section in it. Like some symphonies have fugues worked into in their development sections. You can still call it a "fugue" but its a "free fugue" instead of a "learned fugue".
Or a piece might sound like a fugue but just in the subject entrances, and the episodes are more freely written. That wouldnt be a "learned fugue" either.
"Learned fugue" means the rules are the main focus. It can be a way for a composer to show off their technical skill.
Ive never used the term "learned fugue" though... i think id just call it a "formal" or "strict" or "true" fugue and call the other stuff "fugal"
The use of the word "learned" has changed.
Today we use it as simply the past tense verb of "learn".
But there's an old fashioned use where you actually pronounce the "-ed" as a separate syllable and it is an adjective referring to someone who is well educated.
I think thats the use here.
Its a fugue that shows you have studied the rules and know how to apply them correctly.
So i think people arent recognising it because its an older way of talking. I think when ive seen it used its been in old books...?