I don't recall seeing posts about disappearing markers, but there are a few on disappearing previews. Latest word is that that it may be related to some third-party effects/transitions.
I do have Looks on an adjustment layer. Although the audio track I'm marking is a separate music track altogether..
Off topic: do you have a preview in Looks on a Adjustment Layer?
Yes Ann. It's a known issue with CC and the latest Looks version.
I was advised by Red Giant to use the version before the current one. I'm not at the PC, so I can't tell you the version.
That returned my previews..