I started building in a world yesterday that has seed:1419546585.
When I came in today, I discovered that I had this light bug.
I tried to solve it by logging in and out again, changing the light, removing my conduit, but none of this has solved my problem.
I'm building in Creative.
Biome: river, beach and lukewarm_ocean
Thank you for your report!
We're actually already tracking this issue in MC-149211, 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.