The black outline of stained glass blocks is transparent like barriers, so if you have a fog effect using stained glass you can see your light source at the bottom. But, when you press your function key that clears the whole screen, go into spectator mode, or switch to adventure mode, you don't have the issue anymore.
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Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-187598, which has been marked as Fixed for an upcoming version.
Please use the search function in the future to prevent duplicate reports.
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