Camera indoor film very dark

I do hope you can help me and this is just a matter auf settings:
The camera picture turns very dark as soon as I start taking a film inside a room.

This happens in artificial light with indoor films: When opening the camera app, all looks OK, but as soon as I push the “film”-button and it starts rolling, the picture turns extremely dark (darker than before). The resulting film is very dark as well.

Taking pictures in the same setting is OK, it’s just a problem with films. Has anyone experienced that, too?

I have a first edition fairphone, unfortunately with the 1.8 update installed - so maybe this is due to the broken update?

Thanks for your help,