Python language server and document content completer do not work simultaneously and can be switched by changing MIME types

In Qt Creator when working with a python project, after python-language-server had been installed with the command: "python.exe -m pip install python-language-server[all]" and after setup, it works slowly or doesn't work at all. I tried several files, but its behavior is the same. But if I change the MIME types in the language server settings (e.g. append with an irrelevant type (from the project's prespective)), seems it works properly. The attached video to this report shows the mentioned behavior of the python language server.