Auto-expanding mail folders to file emails not working

Macbook Air (13-inch, 2017)

Big Sur 11.4

If my email folders were closed, I used to be able to drag/drop emails into sub-folders.

So grab an email via mouse, hover over an email folder which would then auto-expand to show any sub-folders nested under main folder so I could drop the email into the relevant sub-folder.

Recently this has stopped working - the folders are not expanding to reveal the sub-folders requiring me to manually expand the folder in advance in order to show the sub-folders in order to drop an email I want to file.

I recently updated to Big Sur 11.4 - not sure if that has any bearing?

Suggestions?

Thanks!