This has been the way crew grade worked since before I started playing VB6 Aurora, back when the big change was that jump gates did not need components anymore and commercial shipyards started at 10,000 tons instead of 1,000 tons like military. We had Gunboat engines and Fighter engines as special cases to use in addition to Missile engines and actual Ship engines.
Ah yes, the good ol' days, when China got nuked in every AAR.
and as Nuclearslurpee said, once you run out, your ship's crew grade will start as negative instead of a positive number.
If that does not happen, then you've encountered a bug.
To clarify: If you have, let's say, a ship that requires 1,000 crew, and you have 500 crew remaining, then the ship will take all 500 of those
and draft 500 conscripts to replace them. The ship's starting overall crew grade will be the average between the 500 trained crew and the 500 conscripts - so depending on your training level, it might
look like your ship has a real crew because the training level is positive, but it will be
less positive than expected so the mechanic is still working.
If you have zero trained crew and a ship gets built, then it will be staffed entirely with conscripts and you will see a fat, ugly -10% in the crew grade column.