COVID almost certainly had something to do with it, but the US isn't the only country that faced lockdowns, nor is it the only country that experienced inflation. Why is it that most other countries' happiness scores have returned to near-baseline since then, while the US is still so much lower?

The USA really didn't have lockdowns, where people were actually forbidden to leave their homes. Apart from a handful of metro areas, what was actualy implemented were feeble 'stay at home suggestions' with tons of exceptions. It was entirely voluntary, and people broadly ignored them (again, outside a few areas). Around me, everyone was still out and about, eating at restaurants, buying their khakis, and basically ignoring that there was a deadly airborne disease being spread around. The only thing that seemed to be adequately enforced were school closures.