It was probably bitten by the other animal in the enclosure. If the gecko that dropped the tail is eating...it will be perfectly fine. You should probably separate the two geckos because of the aggression. Are they of the same sex or opposite sex? Just about all of the fimbriatus and henkeli that come in as imports have regenerated tails. It is a natural occurrence in the wild...whether it be from courting or predators...it is quite common with these species. It is a survival technique for these larger Uroplatus. When you disturb a vertically resting fim/henk, the thrust their tail forward, as if it was their head. Birds, snakes, lizards, and primates always attack the head first.
Also, overly aggressive males seem to attack the tails of smaller males/females and non receptive females. As long as the animal is eating, It will be fine.
No Uroplatus die as a result of losing their tails...some of the species seem to drop their tail (like old bombers used to drop their used fuel tanks on long journeys) right before they die, coincidentally. If the animal is eating, it will be fine.