Summary

If a ticket is created in Jira it should have a priority mapped to it. Jira system still allocates a priority to a ticket if no default priority is set.

Environment

Jira Cloud

Solution

How does a priority is set to a Jira ticket?

If there is no priority set as default under Settings >> Issues>> Priorities, then Jira will calculate a priority dynamically, by trying to get the one from the middle. So, with the below example:

Critical
High
Medium 
Low

Jira goes for High as a system-default choice. There is no way to remove "system-default priority" in Jira, there will always be some priority that is used by default.