Current State:

In a SLA series report when you select the option "JQL from SLA Goal" and choose the value from the All remaining priorities goal, the JQL that the report is considering includes all priorities, showing tickets that match different goals.

This is currently the expected behavior. To analyze it:

1. Have SLAs grouped by priority
2. Have an "All remaining priorities" and a "lowest" (priority, or any priority) goal
3. Create a ticket that will have the "lowest" priority
4. Create a report and filter it in "JQL from SLA Goal" selecting the "All remaining priorities"
5. Save the report

What's happening: this report is showing the tickets from the "lowest" priority goal, but it ideally should't as you didn't select this goal in the "JQL from SLA Goal".
 

Proposed Future State:

Make sure that the All remaining priorities SLA reports series shows only tickets that have the SLA.
 

Alternative:

In this scenario, the suggestion is to use the "Filter by" from the series as "Specific work items" and inform the JQL that includes the priorities that need to be considered.