Leads become stuck in certain Sales Pipeline stages if you have an automated action attached to a disposition tag to specifically assign a lead to a certain stage within the Sales Pipeline. Same goes for if you ever choose "Select pipeline stage" on a specific lead.
To resolve this, edit the automated action that assigns a lead to a stage in the Sales Pipeline to no longer have that particular action take effect and not endure this problem in the future. Rather than re-assigning each individual lead, delete that Sales Pipeline stage, and recreate it. Click the 3 dots above the stage to delete it, and then click "New Stage" in the top right of the page to create it again.
Instead of assigning leads to Sales Pipeline stages, you can simply just use filters on the stage to allow leads to flow in and out of stages correctly rather than making them stuck in a stage.