Title says it all. As a feline origin in the mod, you gain native total immunity to fall damage but once under the effect of Girl Power from Create : Estrogen - which normally allows you to dash - you take fall damage again, even when not dashing.
I don't know on which mod's side the issue could be so I will report it on both sides.

This is a tragedy for us trans catgirls

You're correct. This does sound like a tragedy.

Should be fixable with a datapack? Should be able to override fall_immunity and make it use in_tag minecraft:is_fall

{
    "type": "origins:invulnerability",
    "damage_condition": {
        "type": "origins:in_tag",
        "tag": "minecraft:is_fall"
    }
}

Something like this

Should definitely be part of the mod itself, though. Origins Fabric fixed this some months back