Release NotesSharePoint site collections within shared web applications are removed gradually, i.e. in postponed way.
Fixed IssuesPOA-90627 SharePoint 2010: Gradual Site Delete
Hotfix can be downloaded here.
Use pa_updates_installer (https://kb.cloudblue.com/en/111323) to install this hotfix. Also this hotfix will be installed automatically during upgrade to POA 5.5.8 via pa_updates_installer. To install hotfix manually obtain its distributive from the link above and follow instructions found in file UPDATE.txt of distributive.