You did not approve the edit. You only voted to approve. The edit was still just pending. By then editing the post from the revisions page, you triggered an auto-reject from Community (which has a veto vote), as the suggested-edit-under-review could no longer be applied.
This is by design; if an edit is applied to a post all pending suggested edits are rejected as the revision from which they were based no longer exists. By going to the revision list to edit a revision you are explictly bypassing the suggested edit.
You should instead have picked Approve and Edit. In that case Community will auto-approve the edit (again with a binding vote) and your new edit is applied on top of that edit.
Since that system is already in place, I see no reason for the current behaviour to change. It could be argued that the explicit revision edit is a deliberate mechanism to bypass suggested edits, and auto-approving just because you voted would be very surprising indeed.