When you hold the flashlight and then put it back on your uniform, its light source disappears, and your flashlight ends up inside your body.

Confirmed characters with this bug are:

Regular Rifleman
Regular Light AT Rifleman
Regular Combat Engineer

Another bug I'm seeing is some soldiers don't even have flashlights in their inventory at all, this mostly seems to be for green camo soldiers as seen below.

this includes:

Rifleman

Combat Engineer

I haven't checked all characters or factions, so I'm not sure if any of the others have these issues, especially on US or FIA.

I also provided a video showcasing the light in body bug below.