Yes indoor bonsai trees do tend to lose their leaves more than outdoor bonsai trees.
When bonsai trees are indoors they tend to get less sunlight which causes the bonsai trees to start to wilt and then the leaves fall off because the bonsai tree needs plenty of sunlight to thrive.
Also another problem that causes bonsai trees to lose their leaves is root rot that is sometimes caused by over watering of the bonsai tree.
When bonsai trees are indoors make sure to place them near a window where they can get plenty of sunlight and avoid over watering them and the leaves should not fall off as much.
Still it's possible and normal for indoor bonsai trees to lose their leaves from time to time.