That type of thing is a but irksome.....but what really kills me are the amount of invasives still being sold.

Yes my Vanilla Strawberry hydrangea is really bad about that. The bigger it grows, the worse it splays. After a big rain I try to go out and shake the blooms a bit to get the weight off.

We have a decorative grass - purple fountain grass- that completely flattens in the rain. A day later it’s back five feet tall again.

The vendors are not going to mention any negatives about a plant.

The Miscanthus, unfortunately invasive so I stopped growing them, never flattened. In fact, 'Morning Light', would flatten in winter under heavy snow, but once the snow melted, it straightened right up again. Calamagrostis 'Karl Foerster' a grass I have used in two gardens always falls over in early winter and never gets up again. I leave all my grasses standing all winter except for KF which I cut back pretty early once winter weather sets in.

Put plants supports around them.

They'd still flop down over them unless the supports were nearly as tall as the grasses--ugly and scarcely worth it, and where on earth would you find such huge plant supports? I just let them dry off and stand up again.

But I do think sellers should tell buyers.

That is odd, I have P. v. 'Shenandoah' and it never flops no matter how strong the deluge.

I wonder if cultivars differ.