Unless you are starting from a custom template where you have exorcised all remnants of the default Helvetica Neue Regular 11pt that Pages v7.3 uses, then that will be the default font in new Text boxes, regardless of what current font is used in the document body.
I have a custom template that uses 12 pt Times New Roman Regular. When I start a new Pages document with it, that font is the default font, both in the document body, and in newly inserted Text boxes. Because the Text box is using a transparent background, the text entered in it will be slightly lighter than the body text of the same font hand due to the dithering caused by the transparent background. As soon as you fill the Text box with a white background, it matches the text in the document body.