I started this thread and I kind of found the solution. But since this is an effect of the h5p plugin, I'd like to post my question here. To summarize, it seems that, when deleting the h5p activities created with the plugin, and after uninstalling the plugin, the generated grade items still remain. In some cases, this causes a problem.
My questions are: how has the rest proceeded when migrating from the plugin to the core activity? Did you encounter this grade items issue?
I know that re-installing the h5p plugin and disabling it would solve the problem, but I'd like to avoid having a plugin that has no further use.
Any sharing of the experiences with the migration is welcome!