Japanese Zelkova

I have recently been gifted one of these lovely bonsai trees and have been learning about it. I notice the leaves are beginning to fall and wondered if this is normal? It is indoors, in the window of the lounge. Overnight the blind is lowered so it is not too cold. I have central heating but only 19 degrees, the radiator is not near the window and the bonsai.
Thank you

Perhaps add some photos here?

This plant should not be in such a hot room, it should be outside. You will have to acclimatise it though. Read about Chinese Elm care on this web site in the care guides

The care guide says that it is not frost hardy, wich is wrong.

On the other hand 99,9% of the trees sold as japanese zelkova is really misslabled chinese elms. So most likelly this one is as well.

Here comes the tricky part. The care guide talks about indoors keeping of chinese elm as well. In my experience not possible. However outdoors in usda zone 7 works. If living in lower zone cold but frost free overwintering is the key. Never in livingroom conditions.

Yes, mine are out all the time with only a little protection, for the small ones.