I don't know if it's better to post here or in the To-Do community.
We use Planner from inside Teams. Our Teams often have Guests. When Planner tasks are assigned to native members of our org, integration with To-Do works great - their tasks show up for them in To-Do.
However, the same doesn't seem to work for our Guests. Most, if not all of our Guests are also Office 365 users licensed for To-Do in their own home orgs, however, when we assign tasks to them, they don't show up in their home-org To-Do app. This would be great if it was supported, but I'm not sure if it is and just broken. I didn't see anything in the documentation that says so either way, and didn't find anything in the roadmap or user voice, so I'm wondering if anyone could shed light?