My instinct here is based on that HubSpot does not delete 25k rows all at once, and it can have some lag. So when you went to upsert 25k contacts, 15K of the contacts were created new, but 10k of the contacts were updated and still yet to be deleted, so when the deletion process continued it deleted some of the contacts that had just been updated. This is all just a hunch though but it would explain why it was not reproducable(or at least is not easily reproducable).
Best,
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My instinct here is based on that HubSpot does not delete 25k rows all at once, and it can have some lag. So when you went to upsert 25k contacts, 15K of the contacts were created new, but 10k of the contacts were updated and still yet to be deleted, so when the deletion process continued it deleted some of the contacts that had just been updated. This is all just a hunch though but it would explain why it was not reproducable(or at least is not easily reproducable).
Best,
✔️ Was I able to help answer your question? Help the community by marking it as a solution.