I pay for coffee, who gets the loyalty stamp?

[verybighouseinthecountry]

Sorry for very pathetic question, but I'm doubting myself and just want to know if I'm out of touch and potentially committing a faux pas here.
I have an older friend (60+) who is very likely on the spectrum and does some "off" things that doesn't really bother me as I'm used to her. One of them is when I buy us a coffee, she'll insist on getting 'her' loyalty stamp. (She will always say she'll get us coffee next time but rarely does). I thought this is just her being socially unaware, I would never dream of asking for 'perks' that someone else paid for. All fine. But recently I took someone else out for coffee, I ordered and she sat down, but then ran over to the till when I was getting the loyalty stamp and said "did you get both stamps?!" as if I was a complete CF. Please tell me if stamps are individual, or is the usual etiquette the one who pays gets the stamps?
I'm very aware in the grand scheme of things this isn't a big issue, but I've been robbed of so many stamps if the done thing is to 'split' the stamps I need to know!

Whoever pays gets the stamp.
If your friend trys this again tell her you are getting the stamps because you gave paid.

1. The person who pays gets the loyalty stamp.

2. Stop being a pushover and buying everyone’s coffees.

3. They ask for the stamp because they know you’re a pushover and they can get away with it.

4. Next time you’re out with them for coffee you say “do you want to get these ones as I got the last….and then you ask for your loyalty stamp.” See how they like it.

How unfortunate you have found two friends like this ?? Very odd.

Tell them they can buy the drinks - and get both stamps! - next time

To be honest, I ditched loyalty anythings years ago. If I had friends who needed the stamps so bad, I'd just give them my full loyalty card anyway. If I couldn't afford the coffee I'd buy an aeropress and make them at home [which ironically is why I ended up with an aeropress because 15 years ago I didn't have loads of money to spend on coffee, but now I do, mainly beccause I don't do it very often any more].

My friend and I meet in an independent coffee shop and I have the app to collect stamps and she’s never bothered to download it. We take turns to pay and I scan app.

When we get to the free drink we use it regardless of whose turn it’s to pay as well.

I can’t ever imagine getting that het up over a free drink between friends 🤷‍♀️

CFs like this always know what they are doing. Oddly they don't try it at a petrol station or at the check out at Tesco. CFs could warrant a whole topic on MN. Are they getting worse or have they always been there? The RF are probably the prime example (just an opinion NOT a fact)