FWIW I incidentally feel very similarly. I can write reasonably effectively (if longwindedly), but if I'm particularly invested in a subject I curiously just don't
want to figure out where to start because the whole process feels utterly overwhelming.
A few things come to mind that might be related.
Firstly, (I think that) parts of my thinking/reasoning process route through and leverage the language center of my brain like a crutch, or a like sequence of stepping stones, that my brain somehow uses in order to stay focused and retain my train of thought. I've found that the whole talk-to-the-air/sounding out thing public speakers do has helped me to better elucidate ideas. In that context, I'll talk about something as though it already exists.
Independently of all that, I also find I reason about a few ideas or topics using a sort of indeterminate/open-ended/unconditional focus model that seems to have a bunch of cues associated with it that allow me to carefully isolate the focus process from the traditional "oh meep, solve problem... yay, done!" beginning/middle/end reward cycle, such that I just find myself pondering the ideas on occasion, then naturally drift away when they become boring without associating them with the friction of mental exhaustion in the same way that I tend to do with almost everything (including the discussion topics in the previous paragraph).
Critically, I don't tend to reason about the second group of things using language, instead using nameless compound associations I typically perceive as instantaneous point recollections of That One Time I Thought Of A Thing.
So it seems I have writing and "funnel entire thought process through language center" on one side, and complex problem solving, ideation, long-term focus retention aaaand unfortunately _memory_ on the other side, of a rift I've been puzzling over for years.
(Naturally, if I try to reason about any of those instantaneous recollections, they fragment like waking up from a dream. I've learned to leave them alone, and just try to stumble into them a lot. Not a very effective method.)
This whole issue makes writing about those particularly/exceptionlly-interesting ideas next to impossible: because this rift creates the equivalent of a block against sensible random access to all of those memories, I can only write one-dimensionally about a single element of a given idea that I know I have a much deeper understanding of... and the result is that it feels pointless.
Hm. I just realized this is way more specific than I thought it would be before I wrote it out. (Caveat emptor.) I'm now very curious if this is even remotely tangentially relevant to what you describe.