I use BIOS boot selection to dual-boot. MS has broken it twice. I turned off SecureBoot now and just don't run games that require it.

Apparently you can get a mobo with switchable BIOS config (or was it just a switchable SSD?) so the OS didn't even know that there's a second OS around. If there's no connection of the other OS then MS can't break it [as easily]!

IMO it must be malicious, because otherwise it would be caught with remedial testing. I can't believe MS don't include dual boot setups in their testing.