@bbildman No it doesn't. Click on a folder and then quickly the subfolder. The whole dropdown will close. If you wait for just one subfolder to open up by itself, it will stop doing that even if you click on subfolders afterwards. It will happen again if you close it all and repeat it. I don't remember that happening in 54, otherwise I'd be complaining over it already.
You should edit your opening sentence above to "No it doesn't ON MY SYSTEM" - because ON MINE it works as it did before just like in O54.
Let me be clear, I am using O55.0.2994.37 on a Windows 7 Pro 64 bit, if that will explain further.
When I click on a Bookmark Bar folder, and then click (or hover) on a subfolder (whether quickly or not), the subfolder contents of the subfolder flys out, either to the left or right depending on where the folder is on the BMB. Even if there is a subfolder within a subfolder, the above action is repeated, and the original AND all the subfolders remain in view.