by mobs api, i assume you're talking about the "mobs_redo" mod, if that's it, indeed i am running it, all the mobs redo files are enabled.
On non-ethereal world , animals and monsters have been spawning as they should, it's only on my ethereal world that i have seen nothing during all those days.
Unless it's a "banana jungle" only problem, as i have not yet spent much time on other biomes but after digging into the lua files , i have noticed that may be the explanation :
apparently what i call a "banana jungle" in my "selfspeak" is "ethereal.grove" that is made of "ethereal:grove_dirt"
And if i look at the mobs_monsters and mobs_animal , in their corresponding lua, out of the kitten, none of them has in their mobs:register_spawn line the "ethereal:grove_dirt" location
I wonder if i missed a kitten due to the size of those grasses too, i'll have to re-explore it for a few days.
EDIT : it was a combinaison of that and very likely bad luck regarding the kitten spawn chance.
After walking around a banana jungle again for the day and not seeing anything, i quit , edited the kitten spawn chance from 22000 to 220
And here the little guy came :