I noticed that. I figured the fast assembly would be useful because the assembly project does not stop on its own, but this half project would. You just need to add enough to the queue to max out the population. Of course the bug is that if you over shot, you get to keep it. Not only that, you can still add more even when you are over the limit. I managed to get over 100 population exploiting this.

Mind you, after seeing that, I did some testing and got similar results when I tried to over fill the populations on my worlds using transports.

I think this might explain why yor have just felt strange compared to the interesting balancing act w/ 0.61, I've been finding myself trying to figure out what my cap should be niew rgan a few times & scratching my head when I accidentally assemble too many.  Add in the approval raising ideology structures and I've had more than a few hundred mil+ pop worlds at 100% approval in late age expansion early war. I'd been thinking it was another direction's source