Unless you're holding dye, nothing happens when you right-click a sign, no matter where it is placed.
If you right-click a sign that's on a box/chest while not holding dye, the box/chest should open normally.
(Or maybe it could become possible to edit a sign by right-clicking?)
Linked issues
Comments 2
Thank you for your report!
We're already tracking this issue at MC-112088, so I will resolve and link this ticket as a duplicate.
This is not considered a bug. You are right-clicking the sign, not the chest. For suggestions, please visit Minecraft Suggestions on Reddit.
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Is this still an issue in 1.15 pre2?