Ok, so when I posted a few minutes ago, I tried subscribing to this thread via the controls in the message creation form, and that worked, but I had no problem unsubscribing.

On a hunch I edited one of my old posts in the GIGABYTE thread and unchecked the "Subscribe" box - this did not work. I then tried it again, leaving the "Subscribe" checkbox enabled but in the dropdown list choosing the "Do not notify me..." option.

This seems to have worked, I am no longer showing a subscription and have confirmed it's gone when browsing from my tablet as well. This seems to be a valid workaround for whatever the bug is.