FYI not fixed yet in 8.5 2023-06-12 (build 85550736).
POSTing a string "inserted via Rest API ąčęėį & ž яшерты" results in
"inserted via Rest API ąčęėÄ‾ & ž яшерты" with RFC3986 encoding
or "inserted+via+Rest+API+ąčęėÄ‾+&+ž+яшерты" with RFC1738 encoding, which is the default one in php function http_build_query().