Denys can you share an example string, the resulting URL and the value you expect?
Flarum has 2 transliteration drivers. The "default" driver uses alpha-numerical-dash only and makes use of a PHP library called Portable ASCII, so Flarum doesn't control how it works (We call it through Laravel's Str::slug() helper). We provide the forum default language to Portable ASCII for their locale-specific settings, so make sure to enable Ukrainian and set it as default to benefit from that (I'm not sure if there's any Ukrainian-specific rules in Portable ASCII though).
The second driver "utf8" is implemented by Flarum, but it shouldn't really be able to produce "wrong" results since it preserves almost all of the original characters except spaces.
