Currently, when you insert a content control using TextProperties, CheckBoxProperties, etc., the applied font is the default one for the content controls. Instead, it should use the font of the associated span/paragraph where it was inserted. For example, if you type "Hello world" and set its font size to 36, then place the cursor between "Hello" and "world" and insert a textbox content control there, the font size of the textbox will be something like 10, instead of 36.

Synchronize the font properties and make sure that the content control uses same font as the existing paragraph/span.