DIAGNOSIS New filter media and housing materials are leaching flavor. You didn't prime the filter correctly or long enough.
FIX Remove the filter from the reservoir. Rinse the exterior under cold tap water for 15 seconds. Reinstall. Fill the entire reservoir with cold tap water and let it filter through completely. Dump that first batch. Do this a second time. The third batch should taste neutral.
IF THAT DIDN'T WORK Check the blue filter cap. It must be snapped on tight and fully seated. A loose cap lets unfiltered water bypass the media.
HEADS UP Never use hot water to prime or filter. It damages the ion-exchange resin and can warp the plastic, causing leaks.