If you transferred emails from a previous account utilizing the Gmail import tool, then that account’s labels/folders would have transferred as well.
A Gmail parent label will be created utilizing the previous account’s email address as a label name e.g. PreviousAccount@OldProvider.com.
All folders contained in the previous email account will be nested labels under this parent label.
all my Yahoo subfolders imported into gmail EXCEPT ONE (middle of the list).
What happened?