When the cursor is moved from a part of text that is, for example, plain to a part which is embolden the state change is not notified. In order for the notifications to work, the Bold state must be toggled by the user. This holds for other states like Italic, Underline, etc. too. Each of these states have to be toggled in order for the notifications to work for the specific state.
Fixed now by Henry: http://cgit.freedesktop.org/libreoffice/core/commit/?id=d3a2c0f60365d6092e5061b1d061d6e3e53eb9cf