The A319 is a codeshare flight operated by Austrian Airlines and while LH Group all use Amadeus, you can’t reserve seats on swiss.com on LH or OS operated flights (or vice versa).
At Check-In it works and with calling in it should too.
I’m not sure but you should be able to add your PNR on either LH webpage (to select seats on LH operated flights) or on OS (to select seats on OS operated flights. However I’m not sure if this is possible anymore.