External Recipients are randomly removed from question alerts

Running v0.49.2, Open Source
I have created a question, and added a couple of daily alerts to it. One is to an internal manager, and the other is to a customer rep, and two contacts at the customer.
Everything seems fine for a few days, and then randomly the two customer contacts are removed from the alert and stop receiving daily emails.

None of the internal users are affected. The length of time between when the external users are set up in the alert, and when they disappear is inconsistent.

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what do you mean by customer contacts? can you also upgrade to the latest version 49.11 please

Customer contacts = emails that are not set up as users on our instance. For example, when I put my name on an alert, I see 'ryanr', but for external recipients it's 'person@external.com'

Is it possible someone else is tweeking those subscription?

Do you have access to the Application Database? Can you check the Activity table and check if anything else is tweeking with the subscription setting ... also check the Pulse Table for the Updated At column to check if there where any updates to that subscription

As of version 0.48 those tables are only available on the Pro and Enterprise versions, and we're running the open source.
As for whether someone else is changing the settings, I don't think so--it's happened twice, and it's been addressed with everyone involved in this particular report.
I'm trying to reproduce the issue on a question from my personal space, but no dice so far.

Is it possible that it's automatically removing bounced emails, etc from the subscription? I know the addresses are good, but in communicating with the recipients there was something they had to do vis-a-vis getting them to come through their company's email filter.

nope, Metabase won't listen to bounce emails, as it doesn't have any functionality on the email side other than sending