Yes it would make sense. At the moment an override is set, any changes to default, whether set, removed, or altered, will not get seen. So it does make sense. I do not see a need for this behavior, in that logically changes to default font still have an impact on the system. It's like a list of priorities that, as they change, cascades in a meaningful way. So you can have an override set for the moment, and still configure a default on your mind now that will go into affect later. But, the change you mention would prevent someone from wondering why their default isn't working if they forget they set an override.

Perhaps the best option is to mention that an override is currently set if the user changed the default font. That allows the changes to be made (which is still meaningful), but avoids the user forgetting scenario.