Download attribute ignored on iOS Chrome/Firefox etc

The download attribute seems to be ignored in non Safari browsers on iOS. This minimalistic <a download> example does not trigger download prompt but opens the link in the browser window:

It works as expected on Chrome / Firefox desktop and Android. It also works on iOS Safari.

This is not a cross domain issue. Plenty of similar issues here on SO but they mostly deal with slightly different cases. Also I cannot fix this on the server with content-disposition header (if that was going to help anyway).

Is there maybe something in iOS that prevents non-safari browsers triggering the download dialog?

What's the solution here?

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