Ok, finally found a way to save v17 studio and render nodes apps safely to dl and try Fusion Studio 18. On a Mini M1 as you know...
I preyed hard but :
* it imports PR 4444 - which I know for sure they contain an alpha channel for I also retested them elsewhere before - but doesn't show this very alpha channel properly : as on 17, a black BG seems to be baked in. Of course, you can try any ticking (invert etc etc)
* it can export a PR 4444 AND the parameter "save alpha" has come back in the saver node when choosing PR 4444, but here also, it doesn't take the alpha into account. To wipe any possibility of mistake, I even tried exporting a PSD file (which here shows its alpha channel in the viewer) but no, black BG baked in as if you had ticked "make alpha solid"
In any case, no way to see the grey checkerboard I chose Fusion to show in the viewer for alpha, so I knew it wouldn't work (the PSD showed it properly for example)=> back to 17 and still no option to import and use all these PR 4444 holograms I've been given to work with in my ongoing project