When upgrade from Foxit Reader/Foxit PhantomPDF to Foxit PDF Reader/Editor 11, the customized signatures and stamps disappear due to file name changed. You can find and recover those data with below workaround.
Take Foxit PDF Reader for example:
After updating to Foxit PDF Reader 11, there are two folders under user profile folder, which are:
C:\Users\[USERNAME]\AppData\Roaming\Foxit Software\Foxit Reader
C:\Users\[USERNAME]\AppData\Roaming\Foxit Software\Foxit PDF Reader
Signatures and stamps are saved under InstaSign and Stamps folders, like:
You could just move the folder InstaSign or Stamps from C:\Users\[USERNAME]\AppData\Roaming\Foxit Software\Foxit Reader to C:\Users\[USERNAME]\AppData\Roaming\Foxit Software\Foxit PDF Reader, then restart the software to apply change.