With new bettas, there can always be a whole bunch of reasons for them dying suddenly! The shock of the new tank alone can be the cause, so you should make sure you’re buying them from a reputable source to reduce the chance of this happening.
In regards to flaring, ideally you don’t want them to be flaring for more than a couple of minutes at a time as this can stress them massively.
Here’s an article, giving a complete run-down on new betta fish that you’ll definitely find helpful!