Go to the "plan configurations >> Scheduling >> Maximum assignees per story", the default value here is going to be 3, so even though you have 16 hours for an 8 hour work day up to 3 team members can be assigned to the issue to spread the load of that issue across the associated team members.
You can either reduce the "Maximum assignees per story" value to 1 so that there is only one assignee per story, and you should be all set.
Additional details on the steps for configuring this setting can be seen here:
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