Previously, if you had add-ons installed before you changed the settings to Allow users to install and run only selected apps from the Marketplace, users could still access and use those add-ons without issue.
Unfortunately at the end of February 2022, Google made a change to the way you can manage access to Marketplace Apps and users are losing access to add-ons that are not whitelisted.
What the end-users will see is that the add-on is no longer listed in the Extensions menu, if they try and install it again, it will tell them that the app is blocked.
How do I fix this?
You have two options to restore access to the add-ons. The first would be to add the app to the allowlist. However, if you only want certain people to be able to install the app, you can Admin install the add-on and install it for certain groups or OUs. You will not have to add the app to the allowlist using this option as it is a forced-install.