EM sizes are relative to their parent. In the case of a sub menu the relative parent is actually the body of the site. In other words the sub menu always equals the body font size. In your case that is 17px.
This is intentional as this ensures that a sub menu is always the same size as the body text. Which should be the smallest size font you should use for any important content.
Personally i would increase the off canvas menu item font size to a minimum of 17px. This will make the elements a larger click target for better accessibility. To note – google may penalise you with “Text too small to read” error in the search console if the font is less than 16px.