Read from #1365 - the questions are still valid.
If it's a regression (issue not present in regular nightly), try older builds to narrow down. If that doesn't help, compile from source, running "hg bisect".
Alex, thank you for your reply and providing the link to older builds. Let me answer some questions you asked previously.
Is it a a display issue, or does the image brightness stay fixed as well?
Replies based on Build #26 (Aug 21, 2017 10:24:50 AM) for 6D1.1.6 with no settings changed or modules loaded.
Image brightness does not change when adjusting shutter speed. It's actually frozen in ML menu, but top screen and Canon version of the overlay settings show shutter speed value changes properly.
Shutter speed is set to the following values based on FPS set in Canon video recording settings:
1920/30 FPS → 1/32.43(45) - Fixed
1920/24 FPS → 1/25.52(54) - Fixed
1280/60 FPS → 1/70.03(11) - Fixed
I tried Build #24 (Aug 20, 2017 10:36:50 PM), and it has the same shutter speed adjustment problem as Build #26 (Aug 21, 2017 10:24:50 AM). There is a difference in shutter speed displayed though:

Shutter_Speed_Disabled_in_Video_Mode_magiclantern-raw_video_10bit_12bit.2017Aug20.6D116

Shutter_Speed_Disabled_in_Video_Mode_magiclantern-raw_video_10bit_12bit.2017Aug21.6D116
Build #22 (Jul 3, 2017 12:47:09 AM) does not suffer from this shutter speed issue. So something when wrong with Build #24 for 6D model.

Shutter_Speed_Selection_OK_in_Video_Mode_magiclantern-raw_video_10bit_12bit.2017Jul03.6D116