Facebook Inc. knew of problems in how it measured viewership of video ads on its platform for more than a year before it disclosed them in 2016, according to a complaint filed Tuesday by advertisers.

A group of small advertisers filed a lawsuit in California federal court in 2016, alleging the tech giant engaged in unfair business conduct by disseminating inaccurate metrics that significantly overestimated the amount of time users were spending watching video ads.

The...