Failed An Alcohol Sting Despite Carding The Minor And Examining The Id; Do I Have Any Sort Of Defense?

Location: Iowa

During summers, I work at a gas station, and I failed an alcohol sting today. To give a play-by play:

The (obviously young) minor came up to the counter with their ID in hand. I asked to see it and, after quickly cross-checking the appearance, scanned the ID in... just for it to fail. This is the part where I messed up and where I was looking for advice.

Upon seeing the text, I decided to inspect the card closer. The first thing that I saw was the red bar stating "Under [X] until 6/5/2026," and since everything else seemed to be in order (appearance, holographics, and expiration), I assumed the store system might have a layover period of some kind. The system is janky, to say the least, and I haven't before scanned an ID which was freshly 21.

The problem was that, despite being slightly faded, that red bar said "Under 18" rather than "Under 21" as I thought upon first looking at it. It turns out the ID was slightly outdated - but not expired - and the two indicators are identical in font and color save for the numbers (there's an example of Iowa drivers licenses online).

The primary question I have is whether I have legal grounds for claiming that I did examine the ID in good faith and was simply mislead by poor "graphic design" (for the lack of a better word). The primary thing that doesn't jive with this is that, if I had looked at the DOB rather than red bar, I could've done mental math and determined it wasn't valid. I trusted that red bar, however, and it cost me $800 and my job.

Thoughts and advice?