Illustrator's classic 3D effects have always been unreliable. They generate random artifacts like you have seen. The reason can well be somehow tricky path (maybe an error in the font) or a programming bug. As already said in a comment, adjusting a little the view or light sometimes helps. Font error is not plausible here, because a is ok elsewhere. Expand the appearance and copy the well-formed a-hole.
Your example is not 100% 3D effect. The shading is somehow plausible only in letter r. Letter o is partially right, but it has also extra light which does not look right. The rest is a set of manually drawn straight partially transparent stripes. Hopefully the black parallel lines in the next image help to see it:

The edges of the manually drawn stripes do nor meet the corners of the letters.
The edge lines in the shadings of the holes of letters r and o have a little different directions than the manually drawn stripes. The artist has been in a hurry. As an extra evidence you can compare the original and a succeeded extrusion.

The text was outlined before extrusion. If you have some rounding effect, expand it, too before the extrusion.