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.

There's a very useful explanation of some of the changes - and of things that have always been so but I didn't realise until now - if you click the little question mark next to the pin in the Search bar.