I finally found the insight. The result of my studies is that if a file contains bytes with values >127 pspad refuses to consider it encoded in utf-8. It may be reasonable (I didn't check utf-8 specs) but this means that Utf-8 files, according to pspad, CAN be malformed.
No BOMs involved. What I did to check ouput encoding from my software (php) was to send to the file an extended ascii string (
