If your speakers have their own volume control, then use that. Keep the computer at 100%.
Unfortunately, the consumer market seems to be intentionally designed to make things hard that were not already thought of by the engineers who made each specific consumer-oriented thing. And the previous generation of engineers thought of more things than the present generation that is reinventing the same wheel with far less maturity and functionality. Seems that the goal there is "fewer controls," and, "it just works," which is fine for people who only want to do what it was intended to do, but there's far less flexibility now, to use it for something else.
For example, USB-only headphones that have no controls of their own, and rely entirely on the computer's volume control.
In other words, we have a handful of specific-use tools now, instead of a few general-purpose ones. And if you want to do something different from that pre-concieved specific use, you almost have to become a full-fledged engineer yourself. Try to avoid those well-marketed one-use-only tools if you can, and seek out and buy the general stuff instead. Build your rig with that.
Anyway, for "what happened," the operating system's loopback that it provides for every output device, and OBS uses for the Desktop and Output captures, taps off later in the chain now. Nothing you can do about that. Or it could even be the hardware that changed. Again, nothing you can do, except to know and account for it.
