Ok, I figured out what's happening. It turns out that if a member changes his or her own display name, then it is not logged if that member happens to be a moderator. So this seems to be a major gap that needs filling. It's not only that a moderator changing a member's display name is not logged: it's that a moderator changing their OWN display name is not logged.
Non-moderator members changing their own display name is logged, as you stated.
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