When tasks are grouped to a collapsed sub-process, the link to the generated process becomes corrupt. It needs to be re-added (manually).
However, the corrupt link (if kept) still opens the collapsed sub-process, but launch it in a new window.
Workaround, manually re-add the link.
This is a known issue and has already been forwarded to our developers to get this fixed. If you are affected by this issue and want to be kept up to date about this issue, please create a ticket in our Support Portal and reference the ticket number SPM-12418.
KBA , BPI-SIG-PM-MOD-EDT , Graphical Editor for SAP Signavio Process Manager , Known Error