When the the diagram (stored in SP) in p4b is opened, the cached version and not the current version is opened.
The possible reason is that Office is caching diagrams somewhere. There is a special cache option in Office Upload Center settings (unchecked by default). It may HAVE to be checked for p4b to work properly in order for users to get the current diagram state and not the cached one.
There is also one more option that may potentially affect the p4b function: