I don’t know how your audiocard get’s synced I don’t think it takes it from the Decklink so my best guess is from the graphics card, normally 60hz.
At SVT we use 2.3.3 as video server for the tre largest controll rooms and for a lot off other chains and I nerver heard about syncissues.
The best solution is to use a deembedder to get the correct audio from the sdi where the video lives, they like to stay together. 