Is it normal for a level to read out of true after one bad drop?

The flip test is real, and here's how to make it actually tell you something. Find the flattest surface you've got, a countertop, a windowsill, the bed of your truck. Set the level down, mark where the bubble sits with a pencil on the vial or just remember it dead precise. Then flip it end for end in the same spot. If the bubble sits in the exact same place both ways, the level is true. If it moves, it's out by half the difference between the two readings. So if it drifts the width of the bubble between flips, it's reading a half bubble off, which is enough to matter on stringers. As for the drop, aluminum bodies can take a hit like that all day. What usually gets hurt is the vial itself. They're glass or plastic set in the frame, and a hard smack on gravel can crack them or shift them in thier mounts even when the case looks fine. Some levels have vials you can adjust with little screws, so check if yours has those before writing it off. If the vial's adjustable, you can recalibrate it right there on that flat surface, flip test it, split the difference, adjust untill both directions agree. If the vial's not adjustable or it's cracked, that's usually the end of it. A level that lies to you quietly is worse than no level, because you trust it and find out at the top of the stairs.

One more check that's saved me before: lean it against a door frame or a corner you know is plumb, look at the vertical vial, then flip it side to side against the same frame. If the plumb bubble matches both ways, you've got a level that's fine for plumb even if the level vial took the hit. Since you're cutting stringers, I'd want the level vial confirmed either way before I transfer those marks.

Not a carpenter here so take this with a grain of salt, but I do a lot of shelving in my garage and I dropped my 2 footer off a ladder last fall. Kept using it for months before I realized my shelves were all slowly sloping the same direction. Turns out the drop was the culprit. Now I just borrow my neighbor's to double check anything that really matters, which feels silly but it's saved me redoing a whole wall of brackets. Is borrowing a second level to compare readings a legit way to test one, or would two cheap levels just cancel each other out?

This makes me want to go check mine now. I dropped one down the stairs a while back and just kept using it because the outside looked totally fine, never even thought about the little glass tube inside getting knocked around. So when you say some have screws to adjust it, where are those usually at? Mine just looks like a sealed strip of plastic with green liquid in it, no screws anywhere that I can see, which I'm guessing means it's the throw-it-out kind if it's off.

Been in the same spot of doubt after mine took a tumble off the deck last summer. What settled it for me was borrowing a buddy's little machinist level, the small cast iron kind he keeps in his toolbox. I laid both on the same windowsill and the big one sat a hair off where his read dead on. That told me more in ten seconds than flipping mine back and forth and squinting at the bubble ever did. If you know anyone with one of those mini levels, even a cheap one, set them side by side on a flat spot and you'll know right away if yours still agrees with somthing true.

Update: ran the flip test like described, marked the bubble spot with a pencil on the windowsill and compared both directions. Turns out it was reading different each way, so the drop did knock it off. Good thing the garage shelves guy posted when he did, almost kept using it. Picked up a new one today and it passes the flip both ways dead center.