New custom alien races introduced in aiprops.xml suffer from passiveness that does not effect the standard alien races. Custom alien races often end up moving next to Xenonauts without doing anything. They also seem incapable of reaction fire. I've also noticed that if a standard alien race unit stands next to a custom alien race unit, it kind of activates the custom alien race unit making it behave like it should and be more aggressive. However, if the custom alien race unit is alone or together with only other members of its race, it will retain the faulty AI.
Copying the exact settings of a custom alien race and its ranks into a standard alien race, i.e Caesan, makes them behave like they should, so there seems to be some part of the AI code that is hardcoded to only accept the names of the standard alien races (i.e Caesan, Sebillian, Andron etc). I tried seeing if it was related to the Soldier, Warrior, Elite etc. rank naming convention as well, but it made no difference for custom alien races.
While on the topic of AI and aiprops.xml I would like to request the ability to change the "zombify/spawnreaper" animation and sound effect on a per race basis. The standard animation and sound effect does not fit some of the new alien types I'm creating that use the new "spawnreaper.race.rank.turns" functionality and being able to change it on a per race basis would be of great help.