I'm on Premiere 12.1.2 on a Late 2013 27' iMac running macOS Sierra 10.12.6 with 32GB of RAM. Most of the time, the 'offline' media is inside a multicam clip, although there have been a few instances where it's not. I've seen this on both video and audio clips. The only solution I've found is to re-edit the clip into the timeline.

Restarting Premiere and/or computer also don't fix the problem. pek files does not fix, nor does trashing all Premiere preferences. Making the clip offline and relinking does not fix things, the clip is still offline in the timeline. Match framing from the timeline brings up the online clip in the source monitor. Revealing the clip in project reveals the online clip in my project window. If I right click on the 'offline' clip in the timeline, the option to relink media is greyed out. When I try to export the timeline, I get the 'Offline media is present in this export' error message. The clips are red and will playback and export with the 'Media Offline' graphic. On a number of my projects, the timeline is claiming that clips are offline that aren't actually offline in the project.

Davinci resolve media offline timeline