I've consistently been able to Leash Colossal aliens without first researching Alien Domestication.
There is a Marvel quest that allows this. Could this be what you are experiencing?
I can confirm that leashing Colossal aliens without having researched Alien Domestication is only available if you have completed the
lush biome Strength and Decay marvel quest. If it isn't a bug then it is a bad design decision, because it enables the player to sail around leashing all the Krakens and using them to capture all the water/coastal cities on the map, before any enemy has the capacity to resist such an assault. Also, since leashed aliens don't have unit maintenance costs (another dubious decision), your navy size is effectively unlimited (you may exceed the unit support limit, but since you don't need production to make more units this would not stop the growth of your military might).
-------------
Some other bugs to add to the list:
Moving a water city while it has stationed aircraft may cause one of the aircraft to vanish.
If a city moves adjacent to a resource on an unclaimed tile (ie the move causes the tile that contains the resource to be claimed) and then moves away and out of range, that resource is still considered within the city's range for the purpose of determining whether the city can build structures that require an improved source of that resource within range.
In a case where the above situation exists, if a city is built or moved such that the resource in question is within its city limits, the resource does not qualify the city to build structures that require an improved source of that resource within range - even though the city may be working that resource and no other city may be within range of it.
There is a quest that can trigger when razing an alien nest, which creates a cave in the tile which is then to be explored. When this happens at an alien nest which is in the same tile as a resource, the resource is destroyed in the process.
The AI does not seem to make rational choices about which agreements it makes. For example, on an all-land map the AI is perfectly happy to give NSA free political capital by making the agreement that strengthens the defenses of its non-existent water cities.
The Aquilon unit (a flying aircraft carrier/siege platform) does not have the ability to fire over obstacles, which means that hills or forests that it is in theory flying high above can block its shots. Also, even though it is flying, it can gain the movement benefit of roads and some terrain will cost more than one movement point.
In the hall of fame, all games are shown as having been played in the lush biome, regardless of which biome the game actually took place in.
