I've encountered this numerous times. I would not say it is frequent in occurrence, but it is not unusual to hit it at least once a month. From what I can tell there are 2 manifestations of this.
1. File is saved, when you open the file, symbols are gone. Folders exist, but the symbols are gone.
2. File is saved, when you open the file, symbols exist, but are "empty". So you see many white circles in the scene where these symbols once had content.
I've seen this situation arise in two cases. One when getting work from a freelancer. He could see everything, but I could not. This may be related to him using CS6 versus me using the current version of CC. The second is copying symbols. The general process is:
File 1, copy Symbol Hello
File 2, in library past symbol Hello
Note that Hello and all associated symbols and folders exist. Save file. Close file, open file. Then either one of the two above happens, or it is fine. Which is also key to point it. I can do the process over an over (using the same source File 1 and File 2). It will not consistently happen. But one of the 3 results will occur.
This seems to be a bug in the program that has existed for a while.
BTW, the easy statement here is that some of this is from older versions. But clearly the bug has never been identified, which means its probably endemic to some methodology being used. Also more than likely code from older versions still exist in the newer versions.
All this aside, I've seen another phenomenon where if you paste a symbol that exists in a folder and the existing symbols exist, even if you indicate Replace, it ends up nesting the new symbol in the folder. For example folder is:
After past you see
The workflow here is I having another person work on Symbol1 only. So they do the work for that symbol in an isolated fashion. This works 90% of the time.
As with the above, the workflow now has to be:
Copy Symbol in File1
Paste Symbol in File 2
Verify folder nesting didn't occur.
If occurred, close all files for safety, redo the process
If not occurred, save file, close file, open file
Check to see if symbols empty or missing.
I believe the problem is that the code is buggy around how it handles names. It is possible that other files opened the to have the same names exacerbates the situation. Which makes me wonder if files are "sandboxed". We use Authortime Sharing alot too. The naming method used in the program is terrible. It relies on literal names versus UUID for each symbol. In general using names as IDs are terrible when you have mutation. Essentially you are relying all all code paths as well as metadata being updated of any name change. I would also suspect this is why pasting even with Replace ends up with nested folders.
Thank you for sharing your info. I have shared your file with the team for further investigation. Will keep you updated.
I just realized that you had sent me a link to this post and not the files. Can you upload your files to a shared location such as CC, Google Drive, or Dropbox and share the URL with me? The team would like to investigate.
This is old, but I've hit this problem again. I'm opening a bug report for this. If you or someone else replies, I can provide both a video of it happening and some files which appear to do this 100% of the time.
Definately not solved. It happens to my wife's work every once in a while. She'll open a file and there's an empty symbol where there were eyes, or a prop. It's the worst when it was a symbol that had multiple layers/frames that will need to be re-done (For instance, the eyes).
Same, I just deleted the .fla file so I can start over again. I'm not being rude, it bugs me everytime.
No way we can start a file over. Not when it's 1-3 days of work because of multiple characters, fully animated. Usually, we just copy paste the symbol from our Design file, and override the broken one. Only time that's a problem is if it's eyes, arms, or anthing else that we needed to make a new pose for. Then, it's a matter or re-doing each of those items in the same frame they were in before.