WP 4.8 comes with visual editor type of widgets, a feature that was added without too much thought it seems.
Now it turns out that if a user has the “disable Visual Editor” box ticked in his or her profile, it does not “honour” that setting for the widgets.
If you want to avoid this potential nightmare, then you can install the Classic Text Widget plugin by Christina Arasmo Beymer and Victor Font, which adds a whole new widget to the existing widgets.
Another plugin by Alan Fuller is the Add Paragraphs Option to Text Widget. This plugin modifies the existing text widget by using a filter.
So here you go, if your site uses text-widgets in a way they were not intended and you want to protect your work, you’d better install either of these plugins.
Terrible that plugins like these two are even necessary; one of many signs of the disconnection of WP Core with its audience…