I think I found the solution. I follow the example for this plugin:
https://framagit.org/razer/Octoprint_ABL_Expert/-/blob/master/octoprint_ABL_Expert/init.py
So I created a new class "FilamentHealthCheck" with a get_latest function and updated my get_update_information function with:
python_checker=FilamentHealthCheck,
I hope it helps someone else.
Now the function get_latest is called. I don't know but for me it looks like "python_checker=self" has some kind of conflicts with the main class methods.