Remember GitHub complaining about an anti-pattern? Apparently, it still exists in an edge case!
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Not exactly what the edge case is, but my hypothesis is for attempting to create a PR from a recently dethorned (i.e. I recently changed the default branch from develop to master) branch onto any branch of the same repo (forks don't count).
At least it's just a warning and not an actual error. Like all warnings, you can ignore it.
God bless