(TLDR at the end.) From OpenGroup POSIX specification:

in the locale definition, the expression "[[.ch.]]" shall be treated as an RE containing the collating symbol 'ch', while "[ch]" shall be treated as an RE matching 'c' or 'h'. Collating symbols are recognized only inside bracket expressions. If the string is not a collating element in the current locale, the expression is invalid.

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05

The solution was to use double brackets. I will push upstream to document this better.