I forgot to mention that on the X17 ISOs which seem to cause the trouble, even the Official MS USB preparation tool says that the X17 ISOs are not valid Windows ISOs!
The thing is that it is there as a virtual drive created with the correct files. It has been loaded correctly and I can see \sources folder and Install.wim, etc. but Setup just doesn't see it!
What I seem to read between the lines of the linked post by the good BSD guys:
seems *like* the UDF filesystem *somehow* is a "hard disk device" filesystem (possibly some kind of "hybrid DVD").
Such things may "confuse" different tools, as an example I seem to remember that if you try opening a "same", "normal" (but "hybrid") .iso with 7zip or with winimage or with IMDISK, the MBR or the bootsector of the .iso will prevail:
Additionally it may *somehow* "trigger" a behaviour (bug) of the driver(s), connected with the "type" of disk, something *like* what was found here (or something like "fixed" vs. "removable"):