Hello and wellcome to VOX support forum!
RE: " I wanted this morning to create a Collection (A), but when I added files to it, the ended in the Playlist save as a Collection ("Playlist(1)")…"
Thanks for your report. We are aware of this issue. It will be fixed soon. Sorry for the inconvenience.
RE: "I've also noticed that moving several files inside a playlist doesn't always work (highlighted files doesn't move, but the one playing instead… etc.) "
If you can see the white selection on tracks before dragging - they should be rearranged successfully.
Thanks for using our app. Have a great day!