watchtask callback not firing for the very first time

I have written a custom task as a callback for watch(grunt-contrib-watch).

During the very first time when i edit and save the file, the custom task is not called. But from the second time the custom task is getting called correctly.

Also facing such callback anomalies during watch persisting for long time.

Debugged some files but doesn't get a clear picture about that.

grunt-contrib-watch: "1.1.0"
grunt : "1.6.1"

Start by reproducing the callback behavior with grunt-contrib-watch 1.1.0 and grunt 1.6.1, focusing on the watch entry point and the first file save. Compare the first and later callback runs, including a long-running watch session; done means the callback fires reliably from the first change onward.

Written by the indexing model from the issue text.