The default language for my Office suite is English (UK). When I start typing in a different language, say Dutch, the spellings checkers corrects Dutch text I have entered to a "correct" English word.
After I have completed some part of my text (in general a sentence) the automatic language detection switches the language settings to Dutch and will use the Dutch dictionary to correct my input. It now detect the previous corrections made to be wrong.
In my opinion the proper way would be to first undo all corrections before switching to the new language settings.