FCOM 1.31.40 P27
Indicating and Recording systems - Indications on PFD:
11) CHECK ALT flag (amber)
The CHECK ALT flag appears, as does an ECAM caution, if the disagree between the two PFD altitudes is greater than 250 feet when QNH is selected, or 500 feet when STD is selected.
The caution and flag disappear, when the Pilot's and Co-Pilot's barometer references disagree.
From this I understand that initially the caution and flag both appear but they disappear soon after if the cause of the discrepancy is incorrect altimeter setting.