Answer
This isn't a general upgrade regression — other confirmed upgrades to the same release track show Work Tasks displaying normally, so check the affected user's own access setup specifically: verify they're part of at least one Access Group under the relevant Service Organization, since Work Task visibility and scheduling access are governed by that Access-Group membership; something in the upgrade process may have surfaced (or reset) a gap in a specific user's Access Group assignment that happened to not matter before, rather than the upgrade itself breaking Work Task access platform-wide.