I swear to god there's a meme of Hank Hill checking his levels with other levels that I desperately want to post here.

You need a level checking level.

In all seriousness, they make levels like this, they're just a more high-dollar, specialty item. At work we have a "true table", a tested flat and level metal surface, to check and adjust them.

Edit: Here are some examples

Question: if the bubble is correct on one side, but incorrect on the opposite side, wouldn’t that suggest that one side is misplaned and any attempt at adjusting the bubble part would still come out inaccurate?

I guess maybe I’m not fully understanding the situation?

Well ..... Where to calibrate it then? You'd need a reference. Just a clear tube with water works all the time. But yes, it has some inpractical issues

This might be a stupid question, but what do you calibrate against?

I can't with certainty say that anything in my home is level.

Machinist's levels usually have adjustment screws on each end: https://www.ebay.com/itm/406571322248

Here's one in use: https://www.youtube.com/watch?v=ue5W2fwKb4s

Huh. I need to check mine now.

I fully agree with you. I've got a very old wooden one which has adjustment screws, and I'm pretty sure older "Stabila" brand ones used to be re-calibratable - though it doesn't look like any of their newer ones are. I don't remember seeing adjustment screws on any other levels though. I suppose at least they're "guaranteed level" for 10 years, and they can take quite a beating and still work properly.