This issue has persisted since Bear 1.0. I find myself sometimes having to deal with exotic languages, and none of Bear’s fonts can handle composite Unicode characters well. E.g., note the height difference between the normal “b” and the b with a dash in the pic here.
This is because Avenir (and other fonts used in Bear) doesn’t handle the latter gracefully so it’s taken from another font. My preference would be if we could add our own fonts to Bear, but if you want to maintain control over that, maybe you could add one of SIL’s fonts to the selection (something like Andika). Note how Andika handles this correctly (Avenir Next to the left, Andika to the right):
