High chlorine levels will bleach out one of the dyes used in the TA indicator solution so instead of turning from green to red it will turn from blue to yellow. The test result is still valid but the endpoint is a bit harder to see. You can add an extra drop or two of the thiosulfate (the first reagent) to counter this but it is not necessary if you can detect the blue to yellow color change without problems. Ideally this test should not be done when the FC level is high.