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, clone that Sales Pipeline stage, and then delete it. Click the 3 dots on the top right corner of the stage to clone it first, and then choose the 3 dots again to delete it.
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.