In the linked video, I reproduce and test different variables in the Miner Workgroup ignoring the scanner and proceeding to mine straight down. I also show the modlist at the end.
https://youtu.be/c1io-fvanmo

In the video, I:

1. Reproduce the bug in it's simplest form, 4 Slave miners surrounding 1 Master miner in a Workgroup. The Master Miner is using a Quarry Scanner. Whether Drills or Mining Lasers are used, it does not matter.
2. Test the same setup, but I do not power one of the Slave Miners. The Workgroup correctly follows the Quarry Scanner. The unpowered Slave Miner is then given EU. After a couple layers, the Workgroup fails to listen to the Quarry Scanner, now digging straight down.
   A side observation: The Workgroup seems it occasionally ignore blocks on the edge.
3. Demonstrate that the bug doesn't occur at all if there is one air block horizontally adjacent to the Master Miner.
4. Show that even if the Master Miner has a tube connected to it, it will still ignore the Quarry Scanner. Void pipes are used to remove clutter.
5. Go back to Setup 3, and add the 4th Slave Miner to the Workgroup. The bug will then proceed to reappear, after mining for several layers.
   A world backup in the state shown at the start of the video can be downloaded here, for reproduction purposes.
   Miner Testing - Backup.zip