When I was manipulating a document as a file layer to avoid loss of non-destructive information, I have grouped the layer, and then convert to file layer. As it turns out, the file that is being used as a file layer gets converted into 8-bit document from a 16-bit document. That's something that should not happen. Basically, make a document with 16-bit RGB. Convert the document as a file layer. Go look at the file layer content to see it in 8-bit RGB.
On second thought, it seem to kept the 16-bit info. I have no idea what has happened, but I'm sure I didn't change it to 8-bit. Oh well, not easily replicable or a mistake has gone by unrecorded in my memories. I will report this again if I see this again.