August 2nd, 2018: I've corrected the underlying bug (votes are now only deleted when overriding approval on non-anonymous edits) and fixed the remaining 23 occurrences of this bug across 11 sites. Thanks for the report!
Here's the short explanation:
When a suggested edit is approved, the system creates a revision for that edit and adds a special type of vote to the post. That vote, attributed to the last approver and targeted at the edit's author, is worth 2 points. This step is skipped for anonymous edits, as there's no user to target.
When a suggested edit approval is overridden, the system looks for the last "approve" vote on the post and deletes it.
So the problem here is that when the system went to delete the vote associated with the overridden edit, there wasn't one - so it found the vote from your edit instead. And deleted it.
This has happened 23 times previously on various sites across the network.
I've fixed this particular instance; we'll need to find time to fix the other occurrences (and fix the underlying bug).