The workaround is to delete the ‘defaults.dfl’ file from the defaults folder:
C:\ProgramData\BERNINA\BERNINA Embroidery\<DesignerPlus 9>\DEFAULTS
This will then allow the checkbox to function normally.
Depending on your system configuration, the ‘ProgramData’ folder may be hidden. You will need to enable hidden files first via Windows® File Explorer. Moreover the folder "DesignerPlus 9" in the path may be named Creator 9 or Dealer 9 depending on your product version.
Deleting the default.dfl file will revert the UI to the ‘as installed’ layout and reset the ‘keep me signed in’ checkbox. This requires you to sign in again on the next start.