Amazon this month introduced MRC (Media Rating Council) viewability in attribution. Since January 1, 2019, only viewable impressions based on the MRC standard viewability are eligible to be credited towards a conversion.
MRC standard viewability requires that 50% of the creative is in view for at least 1 second.
Amazon stated this is part of their ongoing efforts to provide advertisers with better and more actionable reporting. The use of MRC viewability in attribution applies to view-through conversions.