It is the defining failure of the category, and it is entirely a moulding decision made before the pair was ever printed.

Two ways to anchor a post

The cheap way is a blind hole in the top of the footbed with the post glued in. Nothing holds it except adhesive, in a wet, warm, flexing environment.

The right way is a plug pulled all the way through the sole and heat-flared flat underneath, so the anchor is mechanical. Pulling on the strap pulls on a flared head that is wider than the hole.

How to check in two seconds

Turn the pair over. A flared anchor is visible on the underside of the sole, sometimes under a thin outsole layer.

If the sole is smooth with no anchor visible anywhere, the post is glued. Ask the supplier directly and treat a vague answer as an answer.

Why heat and water make it worse

Adhesives soften with heat, and these live in the sun. They also live wet, and repeated wetting and drying works at a glue line from both sides.

That is why a glued pair often survives a shop and fails on the first hot day by a pool, which reads to the wearer as random.

What it costs to do properly

Very little per pair, and it is the difference between a giveaway people keep and one they remember for the wrong reason.

If a quote is dramatically cheaper than everything else you have been sent, this is usually where the money came from.