Dashed line (region) shows solid trailing line for null points

[PT10]

c3 version: 0.7.18 (Latest also has this issue)
d3 version: 4.9.1

Following issue (even reproducible in the online Line chart example at https://c3js.org/samples/simple_regions.html)

In my data series I have few trailing nulls. I want to show a dashed line for all the non null data points. But when I use the following configuration, the chart shows a solid line for the null points followed by the dashed line.

Just copy paste the below in the online example and you will see this issue getting reproduced.

var chart = c3.generate({
data: {
columns: [
['data2', null, null, null, 40, 15, 25, -100]
],
regions: {
'data2': [{'start':3}]
}
}
});