Camera sound issue in silent mode

Perhaps your iPhone was manufactured for a country market where shutter sound can't be silenced by law (i.e. Japan).

Regards

Giulio

Please better specify what you mean for "capture sound in silent mode".

Doing what? Taking a video? A Live photo?

or are you talking about "shutter" sound?

Regards

Giulio

Some device are not allowed to mute the shutter sound.

What is the model number?

That device was made for use in Japan and you can't mute the shutter sound.