This was fixed in
MOBILE-3100, but after migrating to Ionic 5 it's broken again.
There is an existing issue addressing this problem in Ionic, but we could work around it in our app by overriding all the default font-sizes. That won't be straightforward, so at this moment there is no way to know how much effort is required to solve this.
As a side-note, when we fix this in our app we should remove this setting. In Android this is already working, and in iOS we could use a plugin to read the system font size instead of requiring users to modify the settings in our app.