Emails erased from server

Hello!
I'm having a strange problem. Some emails that arrive at an account on the server are automatically deleted. I check it with RoundCube, see the email as "unread," click on it to view it, and it disappears. Searching the server, I see the log that the message was delivered to the local mailbox, but there's no record of the deletion.

If I search for it using whm tools, I find the log of when the email was processed, but when I try to view the file, the system can't access it because it doesn't exist.

I disabled SpamAssassin on the entire server and also a forwarder that was configured for that email (to another local account; the email disappears from both). I don't know what to do or how to check further. Any help?

For more context:
- The emails that disappear are from a web form (on the same server, contact7 properly configured).
- Only those two accounts are checked via RoundCube.
- No application like Windows Email, Outlook, or anything similar is used to check either of these two inboxes.
- Emails that disappear are sometimes deleted as soon as they arrive in the inbox, and other times they disappear after a few minutes.
- Some of these emails do stay in the inbox.