The light level of a chest within a minecart, both placed by the player and naturally generating in mineshafts, is set to the light level of the player/the block player is standing on.
Steps to reproduce:
Load into a creative world and enter the command /time set night
place a rail, then place a minecart with chest on top.
move 15-20 blocks away and place a torch or other light source.
Observe that the light level of the chest inside the minecart changes with the player's.
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. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
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