Folders aren't showing up in Outlook, no dropdown arrow

Issue

Below screenshot shows the nav panel in Outlook. In this example, the NRG folder is supposed to have a drop-down, and the Maersk folder is supposed to have more selections under its dropdown, but has only one here.

Solution 1: Disable "Cache" mode in Outlook

When you set up your mail profile in Outlook 2016 -2019, some of your messages and the address book are cached (stored locally on your computer) by default so that Outlook can retrieve the information quicker. This cache can sometimes hinder the performance of Outlook. The program will be slower to process changes in the Global Address List, or will be unable to find certain emails when you search for them. If you are experiencing any of these issues, follow the instructions below to turn off Cached Exchange Mode.

These instructions are for Outlook 2016, but are also applicable to Outlook 2019.

Open Outlook, and from the tabs at the top click on File.
Select the Account Settings drop down menu and then click on the Account Settings options.
From the Account Settings double click on your email address.
Under the Offline Settings header in the Change Account window, uncheck the box next to Use Cached Exchange Mode and then click Next.
You will see a pop-up window reminding you that the changes won't take effect until you restart outlook. Click OK.
The final window will confirm that the change was completed successfully. Click Finish.

Reference article: Article - Turning Off Cached Exchange... (teamdynamix.com)

Solution 2: Run "ScanPST.exe"

Navigate to Outlook's .exe file by Windows Searching "Outlook", right-clicking the app and choosing "Open File Location". That will take you to the shortcut; right-click that shortcut and choose "Open File Location" again. Search for "ScanPST.exe" in that folder and run it against the user's current OST or PST file. Allow it to backup the OST/PST before making repairs once it finds the issues. The program might hang, just let it finish.