When changing the value of a field (ie; Status, Queue, Assignee) for a ticket, the new value doesn't apply.
Workflows are setup in Admin > Business Processes > Workflows for the Service Desk. These can automatically update ticket fields and can do so unintentionally.
First, check the Audits log by clicking the Audits button in the affected ticket. This will show any changes made to the ticket; any changes made by a Workflow will show as being updated by the "Workflow Service", as pictured below.
You can then check the second tab, Workflow Logs to determine which Workflow has updated the ticket. You will now want to edit the Workflow in question by navigating the Admin > Business Processes > Workflows.
Check the Conditions and Update section of your Workflow to determine if this doesn't have ample conditions to prevent unintended use of the Workflow (ie; add a condition that will only assign the ticket to "Ryan" if the assignee was previously blank)
Test this again after making your changes and make sure to Save your work.