Hello. We faced the same problem. If you save Russian files from a letter, it is saved with unreadable characters. A colleague spent a very long time searching for the problem and it seems like we managed to solve it.
You need to make changes to removed link there you need to add the following values ​​to the key name="mailboxd_java_options" parameter

-Dfile.encoding=UTF-8 -Dmail.mime.decodeparameters=true -Dmail.mime.decodefilename= true

As a result, the whole line looks like this:

  <key name="mailboxd_java_options">
    <value>-server -Dfile.encoding=UTF-8 -Dmail.mime.decodeparameters=true -Dmail.mime.decodefilename=true -Dhttps.protocols=TLSv1.2,TLSv1.3 -Djdk.tls.client.protocols=TLSv1.2,TLSv1.3 -Djava.awt.headless=true -Djava.net.preferIPv4Stack=true -Dsun.net.inetaddr.ttl=${networkaddress_cache_ttl} -Dorg.apache.jasper.compiler.disablejsr199=true -XX:+UseG1GC -XX:SoftRefLRUPolicyMSPerMB=1 -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=15 -XX:G1MaxNewSizePercent=45 -XX:-OmitStackTraceInFastThrow -verbose:gc -Xlog:gc*=info,safepoint=info:file= removed link :time:filecount=20,filesize=10m -Djava.security.egd= removed link  --add-opens java.base/java.lang=ALL-UNNAMED</value>
  </key>

As a result, at least we began to save files with Russian names in Russian.