If you have used workspace groups to control process interfaces, you need to create workspace collections to group the workspaces that should continue to have interfaces between each other.
Examples are
If you have used “common elements” workspaces, those workspaces need to be added to all workspace collections that should have interfaces to them.
Please note that updating process modules in and copying elements into workspaces that are not part of any workspace collection, will not result in any process interfaces anymore.
Modelling tip: after the upgrade to 26.1, use the “Process Interfaces” menu to find out about current process interfaces of a particular workspace and add workspace collections accordingly.
As a result, process interface creation should now be much more transparent and easier to control.