There are much better comments here (especially the one pointing out that 'impostor syndrome' may be characterized as a misconception about the level of required competence), but I'd like to add this:

If I understand correctly, "impostor syndrome" is a term coined by ivy league university psych counsellors. It does not apply to the workplace. It refers to ivy league freshmen not thinking of themselves as geniuses and/or mistakenly believing you have to be a genius to be admitted to the ivy league.

That doesn't seem to be your problem though. It seems more like you feel that you're not learning anything permanently. That seems unlikely but... maybe it's true? Maybe your job is all about pointless details of framework X which you then promptly forget again.