No-Break-Space - Is it a Bug or is it a Feature?

If on macOS 15.1.1 in Onlyoffice 8.2.2 you want to enter a no-break-space by ALT+SPACE oder by ALT+SHIFT+SPACE not only a no-break-space ist inserted but a normal space followed additionally by a no-break-space is inserted. Is it a bug or is it a feature?