It's always been that way. Mechanic-rated Petty Officers will switch automatically. Common sailors will switch automatically. Unrated Petty Officers will not switch. I'm not sure why this happens, but it does.
The answer is to use all non-rated POs somewhere else and fill the extra slots with regular sailors.
Or you can use SH3 Commander to make more Mechanic-rated POs. In real life there's no such thing as an unrated Petty Officer anyway. Getting that rating is what makes you a PO.
