This is likely related to Ocelot / Cat changes in 1.14. The TPPets plugins has not been updated in over a year now and was not designed to work with new cats. If the developer releases a fix for this we will update the plugin to fix this issue. In the meantime, it is loosely slated as an "if we have time" project for the development team. There is no ETA on a fix.
@Tholmar was kind enough to write a fix for this. Updates are staged for next restart.
Newly tamed cats will be registered automatically. Any cats tamed while TPPets was broken will register on their own, it just may take a bit longer to happen.