The issue is not just the face, but the area right of the face. The red line above outlines the area affected.
I superimposed another 24 over the top of this one (with an opacity of 30%), to see if the face overall is correct, and it is.
Looking at the two, I am of the view there are only 2 possibilities here.
1. The stamp has been intentionally altered, and without having it in hand, I can't determine any further.
2. Something came in contact with the stamp while it was still wet, perhaps a finger (the oval does seem to be fingerprint sized). The outer ornaments don't seem to be effected, but they are also a lot less ink, and may not have been wet at the time the stamp was touched. The heavy ink in the inner oval was still damp, and when pressed, was smeared into the design, and obscured the face. This seems to be the most likely possibility, given the oddity, and that is what I would label it... an oddity. Fun stamp, if this is the actual issue.