I had this same issue and assumed the worst that I had been hacked.
So apparently in my case, it was caused by a plugin — Dropbox Full Backup.
From their support response
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This user was automatically created through our backup&restore plugin for maintenance service ONLY.
ONLY if there is some error in the process of backup or restore. So that we can ensure our services for you without any difficulties and complications. This user has a randomly and highly strong generated password.
In any case, before we can touch this user, we send an official request to the site owner to get an authorized access. We DO NOT SELL and DO NOT TRANSFER the data of our users/customers/visitors to third parties.
If you don’t need it – you can simply delete this user and deactivate “maintenance services” in plugin “Settings”.
Backup & Restore Dropbox Support Team
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I never received an “official request” for authorization. This happens with the free and paid version.
Full thread here:
https://www.ads-software.com/support/topic/plug-in-created-debug-admin-user/
