Why the view changes after sunset
During the day, a brighter exterior can make a sheer or solar blind appear private from outside. At night, when the room lights are on and the exterior is dark, the balance reverses. Shapes and movement inside may become visible through the fabric.
This is not necessarily a faulty blind. It is often the expected behaviour of an open weave chosen to preserve daylight and a view during the day.
Test the privacy you actually need
Do not judge a sample only under showroom lights. Ask to put a strong light behind it and look from the other side. If the bedroom or bathroom must remain private at night, say that before choosing an openness level.
If the fabric itself is opaque but someone can see around the edges, the issue is fit rather than fabric. Check the blind width, mounting and side clearances with a fitter.
Options that work at night
An opaque blind may provide the privacy you need when fully closed. Another option is a two-layer arrangement, with a light-filtering treatment for daytime and an opaque layer for evening. The right choice depends on the window, room use and desired daylight.
Bring a photo of the window at night and a note of the existing fabric or blind model. We can help compare actual samples instead of making a promise from a catalogue photograph.