Littlegolden This is because Flarums core API does not handle UTF8 atm, flarum/core1949 has the required steps to enable it in the API controller. Something else to note is that the mentions extension will not work properly with UTF8 without modifications. Notably you would need to change src/ConfigureMentions.php
in the following placed:
Usder configureUserMentions
function change the $config->Preg->match
regex to: /\B@(?<username>[\p{L}\p{N}_-]+)(?!#)/ui
And under configurePostMentions
function change the $config->Preg->match
regex to: /\B@(?<username>[\p{L}\p{N}_-]+)#(?<id>\d+)/ui
If you make these changes you MUST run php flarum cache:clear
afterwords.
Reminder any changes you make to vendor files will be overwritten on upgrade. It is never recommended to change vendor files and it is always recommended to use extensions or create PRs to core. When it comes to UTF8 support I am already actively working on PR for core.