Yeah, don't use Default. Ever. Always choose a specific device, *in OBS*.
Default defers the choice of device to the operating system, which has its own logic to determine which device that actually is. That choice can change at any time, and OBS will dutifully follow because that's what Default actually does. And it's entirely possible that different versions of the OS have different logic.
Maybe you plug in a device with audio functions and Windows decides that you want to use it. Maybe you unplug that device and Windows' list rearranges again. Maybe a device with audio functions "goes to sleep" or "wakes up", which is indistinguishable from you physically unplugging it or plugging it in. All of those events can wreck OBS's Default setting, without you having changed anything at all...or so you think because you don't consider a possibly unrelated device appearing or disappearing, and Windows' list of devices rearranging in response to that, and OBS's Default setting taking whatever's at the top of the list at the time.
Default is good to prove that a fresh installation works at all, but it's a pure liability for everything else. Never use Default. Always choose a specific device, so that it doesn't change on you without warning, or choose the wrong one to start with.
