Can you please explain what you mean by process being linked to the activity with screenshots?
Where is the linkage that you mention - stored?
SAP help documentation Integration of EIC and HCM Processes and Forms - HCM Processes and Forms - SAP Library does not talk about FPM forms and could be SAP has missed this.
Since you say the config works fine for Adobe forms I suggest you report this issue to SAP support.
Regards.
