Not really an issue : that button never created a subfolder of the selected folder but a subfolder of the lister current folder.
Nevertheless, I had the same expectation due to the newly introduced foldable/unfoldable folders in Opus13.
What you can do :
Solution 1
Modify context menu for folders (Settings/File Types, then find "All the folders") and add a command (Opus function) either :
CreateFolder : will trigger the popup window to let you specify folder name, or
CreateFolder INLINE "{filepath$}/New Folder" : will create a folder named New Folder and instantly enter inline rename mode.
Solution 2
Enter either of these commands (according to your liking) in a button (with a hotkey if you want) that you will place wherever you want. Mine sits near to the breadcrumbs.