There are some changes came with the recent app updates of the Facebook, and Messenger app. If you check the active friends list area of the messenger app, you will not see the option to turn off or disable active green light for you. In previous app version we had that option there, but it is now moved to the profile section. You have to go to profile of the messenger app, and need to turn on the Availability to make your self visible as online to your friends. But for some people, even if you turn off the availability in the messenger app, still it shows they are online to their friends. you might think it as a bug. But it is not a bug. it is showing like that because you didn't turn off the inbuilt Facebook chat option inside the Facebook app.
So, make sure you turned off it too to hide your online status to your friends, if you want. You can find the Facebook chat option inside the settings of the Facebook app. We have noticed this change only in Android apps, please comment below if you encounter such issue in iOS apps too.