The difficulty command is showing the wrong current difficulty when executed with no parameters, and then refuses to allow a change because spigot thinks the new level matches the existing level (it does not).

/difficulty
"The difficulty is hard"
/difficulty easy
"The difficulty did not change; it is already set to easy"
/difficulty
"The difficulty is hard"
/difficulty hard
"The difficulty has been set to hard"

There are no plugins installed, and I'm unable to reproduce the issue in Vanilla.

Eventually it will cooperate and work as expected if you keep spamming the commands to try and toggle the setting: