I have been plagued by bugs where the symbol breaks for many years.
And finally I discovered how to reproduce.
1) Create fla file of library configuration like image.
2) Place bitmap1 in symbol1.
3) Save the file.
4) Delete the folder1 of the library.
5) Save the file and close it, then open fla again.
6) symbol1 is broken.
This happens when a folder with the same name exists in the parent. In this case it is the aaa folder.
This malfunction is very dangerous. Early correction is necessary.
The team investigated this issue and have logged a bug for it to be fixed in a future update. Thank you for reporting it to us.