If a city is sieged, then units can be put into that city's garrison. I'm not sure if this is a bug or a feature, but it seems to have changed with the recent update. I set up an example of this with my friend. The Drakken player can garrison his units into his capital even though the city is being sieged by Allayi.

Siege Bug Example.sav

Your save and description don't do enough to describe the problem. When I load the save that city just has 3 units in it's garrion and that doesn't change.

Units just can't enter or exit the city if it's being sieged. If a unit or stack of units is standing on a city tile when the city is put under siege, those units can enter the city garrison. Also units in the garrison can leave the garrison and form an army on one of the city tiles then attack the units sieging the city.

What he means is, if the allayi player keeps sieging the city, the drakken armies can use the "transfer units to garrison" army action while standing on an exploitation tile outside of the besieged city walls to enter the city garrison.

You need to load that save as the Drakken player, sorry for not specificing that. It's also easy to reproduce this in a 2-player multiplayer game.

1) Launch 2 player game

2) Walk player 1's units over to player 2's city

3) Have player 2's units nearby, ungarrisoned

4) Wait until player 1 can declare war on player 2

5) Player 1 sieges player 2's city

6) Player 2's units move onto an exploited tile and click the "move to garrison" action

That process is where the save comes from

I have looked into the code and can see why this is happening. However, after comparing the code with the pre-symbiosis one, I find no reason why it wouldnt happen before. Are you sure it isnt just an older issue that has been left unnoticed?

I'm not sure when this was introduced or even if it's a bug (probably is).