How is City Militia / Guardian Statue availability supposed to work?

No idea, it's wonky.  I know they disappear if the city is attacked in the same turn more than once, not sure if they come back or not later.

Its a bug reported before, but to be sure you should try all you can to report the bug again (check the patch notes to see if they fixed i).

Reporting would include either screenshots, or preferably a save where the units are lost.

(check if the units are regained by saving/loading).

Might also want to put up a save before they were lost if able.

Thanks for taking this up.

Sincerely
~ Kongdej

So am I the only one with that problem?? I though it might be supposed to be that way...

I'll see if it comes up in my new game and send saves accordingly.

No, you're not the only one, it happens to me every time a city is attacked more than once in the same turn.  I figured it was Working as intended (not that i think it's a good idea)

In my case they would come back the next turn.

just my last game, I had conquered the city that fielded MASSIVE militia army with some catapult to boot. 

I've immediatelly rushed walls and castle after getting the city, but when the AI appeared I not only did not get the militias AI was able to field, nor did I get militia from the walls and castles, NOR I was able to fight second tactical combat in that turn (I won the first battle) The second army that attcked the city simply claimed it without combat,

if this is the case, I could take all cities with only two units, even weak units. The first one could lose the tactical battle against the defenders, and the second woukd take the city no matter what happened in the first battle. I will try this out to see if it works.

As mentioned, this seems to be the case because the game "kills off" the militias after the first battle and they only restock on the next turn. I'm not sure if this is intentional, but is very easy to cheese (sacrifice a unit to kill off all militias) so I don't think it is. Generally speaking, you should just avoid getting attacked multiple times in the same turn (use tremor to stop one army).

I just tested and attacking the same city twice allowed the city to have all of its defenders both times.

If anyone can provide a save showing this I would love to check it out.

It might be a player-only bug, so you could not test it by attacking an AI yourself. For my abandoned game I certainly did not have Militia units for many turns, even if a city didn't get attacked at all in between.

I stopped because there was no way to win with the militia and reload-finishes-AI-buildings bugs combined, even though I was clearly going to win, without those bugs :/ It's a great game, but a few heavy bugs can ruin it. Still not giving up on it though.

I will try to produce a savegame, but I might have overwritten all of them...

Hmmm... in that case I suggest doing it defending a city, rather than attacking, since that seems to be where we specifically have observed it.

It might be hard to set up a situation where the AI can attack you twice in one turn, but there is another symptom which might be easier to set up: even after a single battle, in the after battle summary all the militia will be marked as "dead" (red icon) even if they actually survived the battle.

I'll try to keep a save if the situation comes up.