Chest minecarts aren't lit correctly in mineshafts. You can easily see this by placing a light-emitting block near the chest minecart, and make sure the minecart is just out of range of the light. Move closer to the light, and you should see that the chest inside the minecart matches the light level of your hand instead of the light level of the blocks around it. This is also visible in third person.
Linked issues
Comments 2
Thank you for your report!
We're tracking this issue as MCPE-22533, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. Please do not leave a comment on the linked ticket.
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I would've attached a video, but it was too big of a file.