I also have the same issue. My mobile icon on messenger and FB isn't visible for my profile and a few of my friends anymore. The rest of my friends are still there and displaying their "Last Active" time. I have updated my phone to the latest software and apps. Not sure why this is like that. I can't find an answer anywhere.
* * * My suggestion is to please submit feedback to the FB team thru your messenger app. If you have the latest update for messenger (Version 2.0.2) its under settings and "Give Feedback". FB can you hear me?? (LOL)
If anyone has any answers to this, please let us know here.