Strange break/place bug

A video is worth a thousand words:
https://youtu.be/IivCqEimeFY

Look closely at 3 seconds in, and you'll see that the block it was planning to jump to broke instantly.

Watching it again, it looks like the cursor was on it while it was sneaking out to backplace traverse, then in the instant the side of the block came into view and it was able to place the bridge block, it broke the block it was previously looking at. what on earth???

Start by watching the linked video, focusing on the behavior around 3 seconds and the described backplace traversal. Reproduce the block-breaking behavior and trace the cursor, bridge placement, and target-block decisions; done means the unexpected break no longer occurs in the same scenario.

Written by the indexing model from the issue text.