Couldn't find anyone who's reported on this issue as well. I've begun hosting a public server and thought the "clickEvent" action would be absolutely perfect for spawn functions.
In place of having to set up a command block hidden behind another block with a button, and additionally labeled with a sign (I.e. for a trade shop at spawn, or to create a random-location teleporter), I could cut out all the other objects in favor of just a sign.
However I've come to find out (only after a few hours of no one telling me) that the signs with commands had no function! They had only worked for me because I was a server operator.
This is very detrimental as I can hardly see any functions for command signs outside of this exact use case (protected server areas).
The only workaround would be to lift the spawn protection or place the signs outside it... which would then open up the potential to said signs being promptly destroyed by griefers or bad players, rendering them effectively useless.
I cannot see this as being intentional, it would be like making command blocks only usable in survival mode.

Steps to recreate:

• Acquire or modify a sign (within a multiplayer server's spawn protection) to run a command (Example command: /data modify block ~ ~ ~ Text2 set value '{"text":"Click me!","color":"blue","underlined":true,"clickEvent":{"action":"run_command","value":"tellraw @s \\"Test!
  ""}}'
• Use sign while being an operator, and observe it is functional.
• De-op yourself
• Attempt to use sign, and observe it is no longer functional.