By putting a standing sign between a support block and another block above, the sign goes through the block above it, making it look bad. It is a totally visual error but it still ruins the gameplay experience.
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Can confirm in 1.18.1.
This ticket actually appears to be a duplicate of MC-3904 which was later resolved as "Works As Intended".
I can confirm that this is an issue. Also affects 1.17.1.