Without having any text highlighted, if you use the mouse to change to Bold the toolbar indicates this action, but you can‘t carry on typing as the keyboard is not reacting anymore, if you mouse-click back to where you were typing the B button remains highlighted but un-highlights when typing resumes and the typed text is not bold. It is the same for Italics, Underline, Strikethrough, Subscript, Superscript, Increase Size and Decrease Size.
It is specific when using Collabora with Nextcloud. It does not appens when using collabora either with Alfresco ou Owncloud.