I have tried to recreate your problem on my Macs and so far everything is working as it should. I'm suspecting you have the IPTC/XMP preferences set to the Date option for the IPTC Info dialog. This setting is called IPTC Info Date and Time uses : and is located at the bottom left of the IPTC/XMP preference page. If you set this menu to Capture Time, then the IPTC Info dialogs will display the Capture Time and Date in gray text.
If you don't want to change the Capture Time and Date when applying the Stationary Pad, then un-check the Date apply box in the Stationary Pad before you apply it to the selected images. This will prevent PM from updating the Capture Time and Date no matter what the Date option is set to in the Stationary Pad. I hope this helps. Please send me more info on how you are able to recreate this problem if it's still present on your Mac.
regards,
--Bob