Shared mailbox hell, how do I get out of it?

We've got a shared mailbox in my org (exchange 2016) that's unfortunately critical. Every couple of days a random user who uses it will suddenly no longer be able to "send as" the mailbox because they get a permission error saying they don't have "send on behalf of" (they do). I've tried everything [here](https://support.microsoft.com/en-au/topic/a-user-who-has-send-as-permission-can-t-send-email-messages-as-another-user-account-in-outlook-in-office-365-920d8ff3-9978-fee5-ac4e-9cca042650ab) and [here](https://www.admin-enclave.com/computer/exchange/resolved-you-do-not-have-the-permission-to-send-the-message-on-behalf-of-the-specified-user), but nothing seems to stick permanently or work for all users. I've also tried a group policy to force online address book and thought it was working, but within 6 hours the user lost the ability to send as the shared mailbox again. Anything else I should try?