The red area is list of blocks that sign can't place & the green area is list of blocks that sign can place
You can't place sign on repeater, comparator, redstone wire, buttons, lever, flower pot, all type of torch & lily pad but you can place sign on lantern, pressure plate, cobweb, turtle egg, pointed dripstone, conduit, cake & campfire
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is duplicated by 1
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Comments 4
While it has been the case since addition, the rules for what supports a sign is rather inconsistent, and I think Mojang should take a look at this themselves.
this is not a bug, this is how signs support to work