That is NOT a bug. It is currently working as intended. You can always artificially increase population by landing loaded colony ships or transports to the planet in question. It was this way back in GCII which allowed you to go over cap. Any race can do it. What needs is  a mechanism that slowly reduces pop as you are over cap.

That mechanism is there as well. The issue I find is that thanks to the assembly build project you can continuously go over. Just drop a point or two of population into a ship. Build a high end assembly. Drop population point down. Profit until the (very slow) mechanism reduces population down to max cap. Repeat ad infinitum.

As Ryat said, yes you can build above the cap, but you have to drop your pop below the cap first.  If you're the tiniest bit below the cap, you can build an assembly project to take you above it.  You can then offload the extra pop onto a transport or colony ship and build another assembly project. 

When you reach your desired pop, you can land the transports, reap the production benefits and send those transports to another planet to continue their nefarious deeds.

Your excess pop will start to die slowly, but the added production can make it worth your while.

Yes, it's an exploit and not a bug.  You could still do it even if they "fixed" it.  You'd just have a lower starting pop to base it on.  

You can do this with the other races as well, but slower growth means it'll take longer.  

Ideally, the "die-off" mechanism should be proportional to the amount over popmax your current population is. If you land 100 pop in one turn, most of them should die on the next turn, then a few less the next turn, and so on.

Something like "50% of the excessive population dies off each turn, minimum of 0.2" would help to get around the exploit.

yep it was, not sure if its broken right now,

but its also only a 0.1 decrease per turn...

its in the GalCiv3GlobalDefs.xml, search for GrowthDecay

changing it to -1 should help with this