A lead gets a Journey status assigned if the company or contact has been assigned to a rep in the past.
The assign button is triggered when there's an open activity on the lead, such as a phone call.
If the sales rep has completed this activity, but has not qualified/disqualified the lead or opened an opportunity, it will look like the lead hasn't been assigned.
We recommend that reps always either qualify or disqualify the lead or create an opportunity on a lead as soon as possible.