Which version of Exchange do you use?
OOF is enabled for internal user by default. I would suggest you have a check on transport rule, whether there exist transport rule delete OOF for internal users.
If you are using Exchange on-premises, you can also use Message tracking log to check whether OOF generated for internal mailbox:
Get-TransportService | Get-MessageTrackingLog -MessageSubject "Automatic reply: Message Subject"
If you are using Exchange online, you can open a service request to Office 365 team.
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