Title basically covers it.
On a Realms server, have a building with a large flat top at world height (256). Mobs spawn on top during the day, and at night even though the ceiling is partially glowstone and the lowest light level displayed in the f3 menu is 11
Thank you for your report!
We're actually already tracking this issue in MC-68565, so I resolved and linked 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.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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