It's not really a problem of the platform, except insofar as you'd like Amazon to only ship from their own warehouses. It's just Amazon reflecting reality on the ground in the EU.
Which is that even if it's legally a single market it's common that stores that deliver something to your door only sell their products in their own native country, or only within their local region.
Aside. I've seen the exact same problems with our local Amazon "competitor". As soon as they became a platform marketplace I've started using them less and less because of the same quality/delivery issues.