Bring your plans to life with guests in Planner

I'm very much liking Teams and Planner in particular, and with the new Admin panel in Office 365, enabling external and guest user access rights and permissions is easy (although it seems to take a loooong time to take effect). 

The issue I'm facing is that guest users cannot see tasks assigned to them in their Planner app.  We have 3 separate companies, all using Microsoft Office 365, and each creating Teams and plans in Planner (with a mix of internal and guest users).  The only tasks that appear in Planner for each user are those that are within plans inside their own company.  All tasks from the other two don't appear.  The only way to view them is to log into Teams for that company, as a guest, and open the specific plan and apply the filter.  This affects the web interface, desktop app and also mobile app.  This makes it very difficult and almost unusable to collaborate on tasks with guests, as they don't know what has been assigned to them or when it is due.

I personally don't have any of my own plans, but am assigned tasks by other companies.  When I open Planner it's therefore empty, and when I try to open any of the plans on my mobile it says the link is invalid or I don't have the appropriate permissions.

Joanna Parkhurst I'm hoping I'm missing something.  Is this a known issue?  Is there a solution or is it being fixed (and when)?

Thank you for any replies and support.