This could be fixed using an alternate approch. Here's a blog with the solution: https://suman-shekhar.medium.com/aem-custom-xmp-metadata-writeback-92f69a137489

In the newer AEM versions (like 6.4, 6.5, cloud), you would find all the inbox notifications under "/var/taskmanagement/tasks" and clean then up the date folders under the node to have a clean inbox.Snip from AEM 6.5: