Thanks for flagging this, and thanks bi85qad for the clear description, and ianfontinell for pointing at the default categories, that was exactly the right lead.
You were both right about the cause. A change had crept in where typing a search term would quietly limit your results to your default categories (the ones set at My > Content). So if the thing you searched for lived outside those categories, you would get few or no results, and picking a category manually was the only way around it. Not what anyone expects when they just type a word into search.
That is now fixed: a keyword search looks across all categories again, like it used to. Your default categories still apply to the plain browse/landing view (that is what they are for), and if you deliberately pick categories for a search, those are still honoured.
No action needed on your end, just give search another try. Shout here if anything still looks off.