Editing a dynamic’s text disables it
When editing a dynamic’s text, e.g. change from 'mp' to 'mp espressivo' or from 'p' to '(p)'¹, the velocity is reset to 0, and the dynamic is therefore disabled.
One has to manually remember the original velocity from the inspector and reset it there after the text change, or (mass-)edit the .mscx file.
¹) which, interestingly enough, cannot be done by the “surround with parentheses” shortcut