Alpha values of 0 are clamped to black

[rexbron]

Posting here for visibility, I have reported an issue where ProRes4444 files with pixels with 0 in the alpha channel are clamped to black.

https://dcpomatic.com/bugs/view.php?id=3014

Alpha of 0 should be transparent to what is behind it, not invisible

[Carsten]

A file like that should not be considered a 'master-file'.

[IoannisSyrogiannis]

Please, consider answering to this as of minor importance, since it has to do with my personal understanding, rather than support of your report:
What "clamped" means? "Fastened", as "fastened with a clamp"? If yes, what "fastened to black" translates to? "Turned black"?
And when you write "transparent to what is behind it, not invisible" you mean that it should be partially transparent? If not, how is it that it will be both transparent and visible? Will it be perceived by a scattering, warping or other effect applied to the background light/image, in the same fashion that a lens works?
Are those two videos intended to be superimposed/mixed, to one another?