It seems that insertable and updatable options are let aside when they are specified in an embedded id. It's what came up from this thread launched a few days ago on stackoverflow.

I was trying to map two attributes in an embedded id to the same column. Though I must admit it's unconventional, nothing seems to forbid it and it was the way I chose to make the things run (working with legacy code and database I tried to do my best achieving my goal). I encountered the problem a second time since, trying to use a column as discriminator, part of an id and join column for a relationship. I know this can sound cumbersome, but again, working with legacy database with no auto-increment ids can lead to that kind of things.

If that behaviour is intended, please excuse me for reporting it as a bug.