

I hadn’t heard of this before, but I think I can see how they’re getting there. With no punctuation, you have to infer the emotional detail from the context. If you add a period, it can be perceived as specifying that you’re using a flatter, less excited tone.
Compare:
Grandma’s coming this weekend
Yay
To this:
Grandma’s coming this weekend
Yay.
On top of that, these kids are used to using emoticons and such to indicate a variety of emotional shadings, rather than just an exclamation mark - so deliberately specifying the flattest, least interested emotional load could very well read as just waiting for the conversation to be over.





Could, sure - this was a fast example off the top of my head; someone else in the thread used “Great” as a better example.
But even here, while “Yay” could be seen as sarcastic, “Yay.” looks even more so