Exchange 2016: Out of office keeps switching back on for one user profile

Exchange 2016: Something keeps resetting out of office for one of our users to on, and restoring the message text even though I've deleted it. I thought it might be something on the user's computer, but this keeps happening even when their computer is off. I've tried turning their out of office off with powershell, via OWA and in Outlook 2016 from the user's computer. Each time after 1-2 minutes the original settings are returning. This is an in-house Exchange on our own AD. Mailbox access is set to defaults apart from when I'm testing I give myself temporary rights.

This started after they returned from maternity leave, so the OOO was set several months ago, and since then I've given the user a new laptop with Outlook 2016.

We do have an in-house copy of Cascade HR which has impersonation rights to all the calendars in order to create holiday records and similar, but I don't think tihs is interfering. We also have Mitel Micollab, but this doesn't behave like this as far as I'm aware and only affects local Outlook.

Any ideas what it might be?