One huge enigma here is the significant range of responses to "vit D" being found in people. All vital empirical observations, which will ( - in the end) be v. valuable to a host of researchers/academics in making sense of what's happening. Nobody REALLY understands this D field fully yet !
Your reply brings another significant point to light: that ample sunshine in some cases does not raise calcidiol blood levels to a particular desired value ( above 50nmol/L or 75 ?), . . . . but that value CAN be reached with copious daily doses ! V. noteworthy !
This CONTRASTS with blood levels not rising AT ALL with high supplementation over many months, which has also been observed ( - albeit in a limited number).
If I come across any info shedding light on your scenario, I'll be sure to bring that to your attention. However, if you are curious enough in the meantime, I'd suspect the best place to field this out to, would be a forum specifically for the "vit D" dysfunction ( - I've not spent time on any of those so far, but believe there is at least one such forum . . . . and there may be two ?).
Glad on this issue, at least Marz you've found what seems to work for you, in reducing symptoms. As larger dose D supplements have only come about since ~ 2010, 10 years ago getting 250mcg/day would have been much trickier !
The era of mass high dose supplement availability is really in its infancy, and is likely to see new insights and developments over the coming decades, clarifying the presently incomplete picture further. We watching and waiting !
Kind regards from an autumnal W Mids.