Defense fleets??

Does anybody know why sometimes defense fleets won't work in EV? It seems to be especially in systems with 3 or more spöbs. In my plugin, I have one system with two /identical/ moons, one's defense fleet works, the other doesnt unless:

1. you demand tribute from it; it will be dominated without a fight
2. you real it
3. then you demand tribute again, now the defense fleet will come out.

I tested extensively with rearranging the order in the syst resource, no luck

any ideas/workarounds?