There are many reasons why a workflow task might not appear in the Inbox app. One reason I always forget to check is quite simple.
Check whether a scenario is configured!
Simply open the Tile configuration and check the Parameter field. If there is a scenario, it would be something like scenarioId=EHS_HS_HAZMAT instead of allItems=true.
If a scenario is in use, you can find more about it in the customizing here:
In a scenario, a restriction to certain workflow tasks can be configured.
More information can be found here:
https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/8308e6d301d54584a33cd04a9861bc52/68c4379a35f644f6b3a73428594d6c3f.html?locale=en-US&version=LATEST
https://community.sap.com/t5/technology-blog-posts-by-sap/sap-fiori-for-sap-s-4hana-fiori-my-inbox-part-1-activation/ba-p/13326175