I suppose that the game (in a part of the code outside of the DLL source available to us) has a list of unitcombats that work (in terms of animations), and any other ones don't work. And all these unitcombats are treated the same way. I don't know if it's a correct assumption, but I don't have any evidence against it. Possibly (I haven't checked that), UNITCOMBAT_NAVAL (used in the vanilla version of the game) will also work when you define it in the database and assign to a unit. This will increase the number of working unitcombats to 15.
The OP says that the 'UI - Promotion Tree' mod (looks like one of Whoward's mods) supports new combat types, but I doubt if it actually fixes their animations. I will check it tomorrow (now I'm going to sleep), but the name suggests that it's just an UI mod and it doesn't have anything to do with unit animations.