/dialog clear will succeed (success value is 1) even when none of the targeted players are in a dialog. This is inconsistent with eg. /effect clear, which fails when the target does not have any effects (or, if an effect is specified, if they do not have that effect).
To reproduce
/scoreboard objectives add test dummy/scoreboard objectives setdisplay sidebar test/execute store success score @s test run dialog clear @s
❌ The command succeeded (and your score was updated) even though you were not in a dialog
On the feasibility and consequences of a fix
It is probably not very practical to fix this inconsistency since to my knowledge the server neither tracks nor makes any assumptions about players' dialog states. If, despite this, the issue were to be fixed somehow, it would create a situation where players can effectively query whether a player is currently in a dialog but not without clearing that dialog. So calling this Won’t Fix or Works as Intended is totally reasonable, but if you’re up for a challenge, implement the subcommand /dialog query as I outlined in this comment on the Feedback Discord and then consider whether it is possible to do something similar for /dialog clear but where the request times out at the end of the tick instead of after several seconds. Or, of course, you could implement a system where the server keeps track of what UI the player is in just so you can fix this little output message.
Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
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