Why do you suppose it is that every vacuum bottle or such thermal device I’ve tried inevitably declines in its ability to keep things cold?

Such insulating vessels are designed with a vacuum layer, that prevents heat transfer through conduction.
I would postulate that over time, air slowly diffuses into that vacuum chamber, robbing it of its insulating properties.

So it’s designed to eventually fail?

Less designed to fail, more an impossibility to create a vacuum that lasts forever.

Have you tried the Coldest? It’s not cheap but does work well

Thank you! I’ll try one next.

Sounds like a curse.

I’m guessing the facory was built over a mafia graveyard.