Diagnosis
This is the 'break-in' period for the new carbon filter. Carbon fines and initial air in the system can give the water an odd taste, and it removes chlorine which can make some people perceive the remaining minerals as 'flat'.
How to Fix It
Flush the system thoroughly. Run the dedicated filter faucet at full flow for a minimum of 10-15 minutes to clear all carbon particles and saturate the media. Discard this water. Taste should normalize after a few gallons.
If That Didn't Work
Ensure you removed all plastic packaging from the new filter cartridge. A forgotten protective film or seal will severely restrict flow and taste.
Heads Up
If you're used to chlorinated tap water, the filtered water will taste different—specifically, it won't taste like chlorine. The 'flat' sensation is normal; you're tasting the actual water. Give it a few days for your palate to adjust.