Is ‘Keep the Change’ No Longer a Thing in Michigan?

Okay, I'm just going to warn you now that this is going to be one of those stories I tell that makes me feel old.

Now that I've got that out of the way, I have a question;

Is the phrase, 'keep the change' no longer an understood thing in Michigan?

I was getting a coffee over the weekend at a coffee place that had a drive-thru. I was able to pay in cash and offered the person at the window more than my total and told them to, "keep the change". To which they ask, "keep the change, what do you mean?"

Read Related: Unpopular Opinion—The Customer Isn't Always Right

I explained that they could keep the change for a tip, at which point they made a sound like, 'oh, okay, I learned something', they said thank you, gave me my coffee, and I went on my way.

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Am I off here? I though the phrase 'keep the change' was an understood phrase, particularly in a place that has a place to accept tips.

Do you think this could be a generational thing? I'm someone who's in their mid-30's, and I would estimate the person at the window to be in their young to mid-20's... is that something that just isn't said anymore? Could it be something that's going by the wayside because of cashless payments nowadays?

Whatever the reason, it was an interesting interaction, and frankly, I'd love some insight.