Apologies if this has been posted before, but in my years (and years) of playing MOO, I've never seen something like this.
I've been conquering my way through the galaxy, my tech outclassing all my enemies in ground combat and my production fast enough to turn stolen worlds into fortresses before their original owners retaliate. I turn everything I take into a huge Gaia paradise, max out population and industry, and move on. But I've encountered a problem.
On a few of the planets I've taken, population has suddenly and inexplicably dropped to zero. No monster attack, no planetary event, no enemy fleets, no nothing. My colonies are still there--not only are all my fortresses and bases still there, but I also still have (pointless) control over spending ratios--but no colonists. I send transports from nearby worlds, and I'm told that I've founded a new colony. Everything's fine for a few years, and then BOOM. Zero.
So what happened? Have I hit some kind of bug? Am I doing something wrong? It's a Huge map and I've got a far greater population than anyone else--have I hit some kind of ultimate population cap?
Screenshot attached to prove I'm not hallucinating.
EDIT: It only seems to happen when I max Ecology spending. For reasons I can't comprehend. Help?

I found this in an FAQ
"3.C.1) The Biological Weapon Bug: Bringing ships with Biological weapons into a system will reduce the population, even if the ships retreat immediately or are destroyed before ever coming near the planet. Found in multiple versions (other symptoms include biological attacks even when the Bombing option is cancelled if bio weapons are present)."
[url=]http://jonsullivan.com/misc/moo13.html#Divide[/url]
Could that possible be the problem?

Alas, no. There is nothing in orbit over these planets, and I myself do not build ships with biological weapons (the diplomatic cost is almost never worth it). Good eye, though.

The only thing I can think of is that perhaps it is a rolling over issue with the population? Perhaps the population goes negative if it gets too big or something. You did say you were maxing out all your planets after all. I am not sure if this is an issue with MOO (it doesn't ring a bell), but... *shrug*
It is a low likelihood of being this, but since nobody else has put up something, I figured I would throw it out there.
EDIT:
I found this out with a fast Google search. It is a different patch that what GOG is using, and supposedly it has been fixed. However, it does explain your troubles, perhaps.
"3.B) Bugs from version 1.2 of Master of Orion
3.B.1 The Maximum Planet Terraforming bug: Some planets (including Orion) would stay set on Terraforming even after reaching 300 million in population. Increased spending could result in reversion to the base value for the world's population, or even wiping out the colony. This is fixed in 1.3."
It is an excerpt from this FAQ. There are some other bugs listed there, if you want to look through.

Ahh, so it is. I was on the phone while Googling and posting the edit, so I admit I was a little distracted.
It has been around 15 years since I played MOO, but that bug is the only thing I can think of that would do this. I think I even recall it happening to me a few times (probably in v1.2). If it isn't this particular bug rearing its head again, I am at a loss to explain what the OP is seeing.
EDIT:
Unless it i some weird bug with removing the Ship Recognition copy protection. I really don't think it is that, but I just can't think of anything else. I don't even think the game acts this way if you fail the copy protection; it messes with you in other ways, but not this, IIRC.

Thanks for the good research, guys. I've largely averted it by watching what I spend on ecology. It sounds like something that's been patched out, but who knows? It's very odd.