HrefLang / russian language error

Hi there,
PolyLang is a great tool, however I came upon an issue I got stuck with – hreflang annotations for Russian language version, which are automatically generated by this plugin.

Example:
URL: https://wolften.pl/ru/никель/
it is transformed into https://wolften.pl/ru/%D0%BD%D0%B8%D0%BA%D0%B5%D0%BB%D1%8C/ (which corresponds with canonical URL), while hreflang has a value of https://wolften.pl/ru/%d0%bd%d0%b8%d0%ba%d0%b5%d0%bb%d1%8c/.
It seems it is just the question of capital/small caps, but it gives me an error that there is self-referencing missing and hreflang and canonical do not match each other.
There are no issues with other language versions.

Any suggestions how to solve this issue? Quick help would be really, really appreciated

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