Just to make sure I understand correctly (Since I don't speak Turkish).
The issue is, that on page https://tr.wikipedia.org/wiki/Kategori:Bat%C4%B1_Asya_%C3%BClkeleri the subcategory Irak is currently listed under ı U+0131 "LATIN SMALL LETTER DOTLESS I" where it really should be written under I U+0049 "LATIN CAPITAL LETTER I"
So at first glance, it looks like (IcuCollation.php line 397):
// Primary collision (two characters with the same sort position). // Keep whichever one sorts first in the main collator. $comp = $this->mainCollator->compare( $letter, $letterMap[$key] );
Has the comparison reversed, since lowercase comes before uppercase.
However, changing this would probably affect which letter is chosen as section header in several langauges like: dsb, et, eu, fa, fi, fo, fur, hsb, kk, kl, km, ku, ky, lkt, ln, lt, lv, [I stopped checking at this point]
So probably safer to leave that as it is, and special case Turkish.
This will be fixed on Turkish Wikipedia next week, with the deployment of MW 1.33.0-wmf.19.
(No updateCollation.php run is needed; the ordering was already correct, just under the wrong heading.)