Configuring preferences and options in Outlook on the web for business
This may not be relevant to you, but this problem only happens on my laptop.
As mentioned above, I fixed it for me by changing my laptop browser's view zoom level to 90%, or less (you might need to use a different zoom level to me), and it can now continue to display the folder list after I reveal it via the three little horizontal lines in the top left.
Once I swapped to 90% the folder list appeared. I could then change back to 100% and it stayed in place! Go figure :-)
in the left side bar to show or hide folders.