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Why the ban hasn't been reported in the
modlog?
As far as I know, the modlog doesn't report the ban only if that's temporarily.
According to the rules, since
chaser15 (the hacker) has shared a malware, the ban should be permanent. So, it should be displayed in the modlog. It's weird.
Check OP's screenshot - it wasn't permanently banned, it was locked (there's a difference). Also, not all permanent bans show up in the modlog:
Autoban is permanent, unless manually removed/modified. Nuke bans and removes all of a user's posts (spambots usually). Manually applied bans won't show up in the modlog, those are usually temporary, but not always.