On one of my test project I've noticed that the "Subtask" is one single word.
Kindly have a look, it's TEAM managed project:

Another project got this type of issue like following: "Sub-task". It is COMPANY managed project:

It could be I've created this type, and simply forgot about it ^_^
Or this is the Jira settings by defoult?
Could you help me to get it?