Russian layout in VNC to Linux

Hello, dear support team!

What was expected:

Connecting to Linux host via VNC from guacamole, changing the language to Russian (just to be sure that was made in the remote system and in the system accessing to guacamole), trying to type some text and get all the letters/symbols that I pressed on the text editor/console.

What went wrong: some keys do not type any sign.

Diving deep:
Tested on different OS (windows 8.1 and 10 and CentOS 7) from different browsers (Google Chrome and Mozilla firefox) on guacamole 1.0 and staging-1.1 (build of May 2019). As VNC hosts were used RedHat 7 and CentOS 8. Of course, it was tested with VNC client (tigervnc) to test if this problems exist on server side and there were no problem.
What is interesting that it's always 7 keys that are not working in the lower case (In upper it is just one key). That is very interesting becasue in cyrillic alphabet 33 letters and in english 26 and 7 makes the difference between this numbers. The missing letters differs if the different layout of russian were chosen so there can be an assumption that the problem is something there. xev shows no key press when the missing letters are pressed.