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There doesn't appear to be anything wrong.

Moderators are volunteers and aren't required to be on the site all day everyday.

While being missing for a month could be seen as a problem, if the other moderators can handle the site then it isn't currently a problem.

If the absence extends to 6 months or the workload becomes too great for the remaining moderators to handle then the moderators can discuss the situation with the community managers to see what steps need to be taken. That may be appointing a new moderator or even, in extremis, removing the diamond from the "missing" moderator.

So, unless you think that the moderation of the site is suffering because of their absence there's nothing that needs to be done.