Hi.
With the latest version of the Fire TV app (Beta: v2.0.40a (2021/08/24) (probably also with previous ones) I don't see any additional parts (Disc 2).
I use filename "xxx - Disc 1" and "xxx - Disc 2" etc.
In the web interface, "Additional Parts" is correctly displayed here, with the Fire TV app I only get to see the contents of the folders "behind the scenes", "shorts" and "specials" all under "Extras", Disc 2 is missing here.
Same problem with last Android TV App, here al