Hi,
I just added Alex's mobs to my server running in 1.20.1, I don't have any other mods related to mobs or terrain generation, but I struggle to find the majority of mobs.
I find a lot of flies, cockroach, gazelle and sugar glider, but I don't think I found anything else. I checked the config file and every value is at it's default value, all the mobs are enabled.

I read another issue where it was said that it's a normal behavior for already generated chunks, but is there a possibility to change that by resetting the chunks in a way ? Or do you have another solution ?

Thank you in advance, and congratulation for the mod, it's incredibly well made !

[UDPDATE] I used mca selector (here) to delete all the chunks I could in my world and it worked !

Deleting your chunks can be scary but if you made a backup there's no risk, you can find a good tutorial by Wattles for a little help (here).