As can be seen at the following, the 'site' has a case in the general chunk, and each namespace can have another case. e.g. User & MediaWiki & Special are not case-sensitive, whereas the other namespaces are.
https://en.wiktionary.org/w/api.php?action=query&meta=siteinfo&siprop=general|namespaces
pywikibot (core and compat) only fetch and use the case rule from the general site data, not for each namespace.
Version: core-(2.0)
Severity: normal