If you have a brain injury, head trauma, stroke, cognitive migraine.... You're aware of the space you vacate. You can feel it missing and its "a thing".
For everyone else, you just convince yourself you're just as effective as you were. You think multitasking works or driving after two beers is ok. You're not aware of what is outside of your constrained cognition.
Used to multi task like crazy when younger. Turned out it was a coping mechanism for severe ADHD.
Then had a few years of brain injuries.
I would see a problem. Say “reverse a string”, and think oh that’s trivial. A week later I would finally finish. I “knew” how to solve the problem, but as you described something was missing.
Eventually recovered, plus got on adhd medication. Now I don’t multitask much. Only if I skip medication.
I still can but it’s harder unless I’m off medication.
In general I’m much more productive. But grander problems are a little harder as I’m not thinking too much beyond the moment.