WWVB antenna interferance from AC/DC converter supply
You will need to determine what switching frequency the SMPS is using. WWVB uses 60 kHz, and the SMPS probably only operates that low if it operates in a pulse skipping mode with light load.

If it does operate at 60 kHz, you could increase the loading on the SMPS (which is wasteful of power), but otherwise you won't be able to filter out its 60 kHz from the receiver.

Else, you need a lower bandwidth filter on the receiver. Often a 60 kHz crystal resonator works very well for this.