Clips of a frozen track go offline in an apparently random fashion, without any reason.
The clips are not properly offline since the frozen clip name disappears at all, there are not reported offline media and it's impossible to relink them. The only solution is to unfreeze and freeze the track again.
I don't know how to reproduce the problem, but it's been happening only on freezed Instrument tracks (it never happened on frozen audio tracks). I'm having this problem on Pro Tools 2018.1, running on Windows 10 version 1709.