All you need is a 3DEXPERIENCE ID.
I've had this issue for quite some time now, but never really followed it up as it isn't very prohibitive to the work I do. over time it has become quite annoying.
Selecting multiple files (4 or more) in Windows Explorer/PDM, right-clicking and selecting open will never actually result in all files being opened. It seems to be random as to which files out of the selection will actually open, sometimes