I don't do much with PDFs but I know there are issues with updating the metadata in them. It is to do with how it is stored and whether calibre or other tools can update that.
But, if updating the title in calibre works to be seen on the device when you send the PDF to the device, then I would have expected using a metadata plugboard would work. And even if it didn't, the driver should be able to update the metadata on the device when you reconnect. If the driver is configured to update the metadata on the device, this will be checked each time the device is connected.
What does the metadata plugboard look like? Metadata plugboards are specific to the format and device. Is your plugboard for PDF or using "any format"? And set to the correct device? If you are using the KoboTouchExtended driver, the plugboard needs to be for that, or for "any device".
I have just tested this with a plugboard set for PDF and the built-in KoboTouch driver. The template was simply:
This updated the title when I sent the PDF to the device, and updated the title for the PDFs on the device in the same way.