I have 24 citizens on Iconia, playing as the Yor. I schedule pop assembly, of which just one a turn can be done, and when I check back later it still has just 24. My population cap lists as 62, but it just refuses to add any more citizens. No error is provided, and it will spend multiple turns going through the assembly orders at a rate of one a turn, and simply fail to add any new pops.
Two other worlds were able to exceed this and go to 25 or higher population counts. Neither were precursor worlds.
I suspect this is some implementation of the synthetic pop count limit from galciv3, but without associated UI or notifications to the player know what is going on.