Sorry, that is working exactly as expected.
The Series tab on the device is not built from the displayed series name. There is another column in the database that is used for this. For books purchased and downloaded from Kobo, this is set. For sideloaded books, this has to be set by some other methods, such as the driver. The code to do that was added after calibre 3.48. The KOBOTOUCH348 driver does not add this function. It only adds the ability for calibre 3.48 to recognise the Kobo devices that were release after this version of calibre was released. It does not add support for function added to the firmware after that time.
You could try using NickelSeries. This is something added to the device and updates the metadata from the books. From memory, it will update the details needed for the Series tab. It is discussed in
https://www.mobileread.com/forums/sh...d.php?t=331680.