Facebook Messenger Webhook Triggering on Incorrect Page When Messaging Between Linked Instagram Account

Question

I’m experiencing an issue with Facebook Messenger Webhooks. I have two pages, A and B, both linked to the same Facebook account. Page A does not have the message webhook subscribed, but Page B does.

Here's the scenario:

Instagram account a is linked to Page A.
Instagram account b is linked to Page B.

When I send a message from Instagram account a (linked to Page A) to Instagram account b (linked to Page B), the message webhook is incorrectly triggered for Page A, even though Page A does not have the webhook subscription.

I expected the webhook to trigger for Page B, since that’s where the webhook is subscribed. What could be causing the webhook to trigger on the wrong page, and how can I resolve this?