I can definitely relate to this. For the first 2+ years I could eat anything, but from March 7th last year suddenly my body reacted too dramatically to fibre! I don't so much get brain fog from it just a sense of "creepiness" when I wake up in the morning. It's hard to describe. And exhaustion, and gut disturbance (now some reflux)
It was very hard for me to admit that "healthy" food made me worse. Healthy food has been my normal diet for....like, 40 more years! I always ate at least 5 portions of fruit and vegetables a day....pulses, bran, everything.
Suddenly on March 7th I could only rarely even eat a lentil stew, and had to cut down fruit to a bare minimum.
I tried low FODMAP. Some foods even on that list were too high in fibre for me but some were okay, But my diet narrowed down and it concerned me.
So anyway, as a low fibre experiment I ate white toast, hard boiled eggs, white rice, peeled mashed potato, steamed fish, a few lettuce leaves, a few green beans and nothing else (against my ingrained feelings about needing to "eat a rainbow" and not knowing what to do without healthy food etc.)
And I did find I felt better, clearer somehow, gut not so disturbed, not so anxious when I woke up in the morning, better quality sleep, more steady, not as creepy.
And I still can't get my head around it..... about possible malnutrition....a restricted diet....it making no sense..... (I take a multivitamin/mineral complex and a low dose B complex.)
I try to incorporate my more normal foods from time to time, but I can't get away with it for more than 2 days at a time.
The weird thing was, I did 3 gut stool tests and there was no sign of anything wrong. But my tolerance for any form of fibre has decreased a lot. If I eat it -I get effects. Sometimes I eat it anyway because I can't bear the dietary restriction. And pay the price.