I just loaded up the 1.16 update and I used the bed to fast-forward to daytime.
All the maps on my wall were very dark and the whole lighting around them was wrong.
After placing a torch the issue is resolved.
This is definitely due to the 1.16 update because I've had these maps for quite some time and it's first time I see such an issue.
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Update: This happens when I return from the Nether to the Overworld during the night.
It's not related to sleeping (it happens during the night as well) It's just more noticeable during the day.
It seems to affect all transparent blocks, not just maps.
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Thank you for your report!
We're actually already tracking this issue at MCPE-69037, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Yes, it seems to be the same issue. Sorry for the duplicate. I didn't realize it's the same issue.