Y axis formatting - percentage issue

Hello,

I have strange situation with Y axis formatting on line chart. Report contains metric as following:

Main Metric = DIVIDE( [Nominator $], [Denominator $], BLANK() ) 

Main Metric was formatted in the following way:

There are following line charts. In this example all Y-axises are formatted correctly.

The issue is visible if user select wider time range and Denominator equals BLANK for some months. Then Y-axis for Main Metric is formatted in incorrect way (as a Whole Number, not as Percentage). 

I understand that Main Metric for CY B on May and Jun equals BLANK because of Divide by blank. However, I don't understand why Y-axis shows Whole Number instead of Percentage.