If transformative weapon is disarmed while in alternative form, would this mean it transforms into its original form, due to "When unattended, the weapon reverts to its true shape." clause?
Yes.
If you had a medium longsword with the transformative property, and then change it to a scimitar (or whatever), and then it becomes disarmed or dropped, it reverts back to being a medium longsword.
Makes sense. Ty.