In the last couple of weeks we received several support cases (and comments on my blog) that after applying May 2022 CU the left navigation menu on modern pages in SharePoint Server 2019 is not showing up.
The issue is currently being investigated by the product group but there is also a very easy workaround to prevent the issue from happening: enabling the side-by-side functionality.
If the side-by-side functionality has not been enabled before, the following steps are required after installing May 2022 CU:
1) Verify that the version specific side-by-side directory has been created in the layouts directory
If this directory is missing you can create it using this command:
2) Set the side-by-side token to the version number reflected by the directory
3) Enable the side-by-side functionality
An IISRESET might be required to ensure that the new settings is active.
More details in the my previous articles about Patching using the side-by-side functionality.