There is a bug in my path generator algorithm that happens when you have a hole inside a shape inside a hole inside a shape (oh mouthful), which the holes inside circled 6/8/9 happen to be. I can’t remember if it forgets to insert a path or traces it in the wrong direction (CW/CCW) but either way the font renderer is not amused.

If these are the only such glyphs in the font, you could make a hole in the outer outline and then move the vertices in FontForge when you’re done with the font. Or put the circle in another glyph and then merge them using FontForge (possibly through a script if you update the font often).

Or, if you like a challenge, you can find try fixing the algorithm (source code’s here).