I am using Nuendo 13 in Windows, but also had this problem in 12.
When opening a project that has existing Spectralayers 10 edits on clips as an ARA extension, random clips lose their state. No audio is produced, and the clip shows just blank space in the Spectralayers editor. This doesn’t happen on all the clips, and I can’t seem to pinpoint any reason for some clips to disappear while others remain available.
I have to remove the extension to be able to get the audio back, and then start over by re-adding the extension and performing my edits again from scratch.
I have read other reports about this problem with Spectralayers and other ARA extensions in Nuendo and Cubase, but I haven’t seen any solutions.
I prefer not to have to make extensions permanent before exiting my project each time because then I lose the ability to modify the edits later.