Measure showing value for periods without data

Your last bit after the return 

RETURN (1 - DIVIDE(cntDefects, COUNTROWS('Audits')))

is evaluated to 1-blank(), which PowerBI translates to 1-0 = 1 => 100% 

You can test this directly with

Minus Blank() = 1-BLANK()

which also returns 1.

If you want those entries to disappear from your visual, you’ll need to add a check before the subtraction and return BLANK() whenever the denominator is empty.

For example:

~Defect % =
VAR cntDefects =
    COUNTROWS (
        FILTER (
            'Audits',
            NOT ( ISBLANK ( 'Audits'[QAIssues] ) )
                && 'Audits'[QAIssues] > 0
        )
    )
VAR cntAll = COUNTROWS ( 'Audits' )
RETURN
    IF ( cntAll = 0, BLANK(), 1 - DIVIDE ( cntDefects, cntAll ) )