I have a simular support case open at Poly support. it all started with a Poly Lesn group lost it members.
It turned out that the changes in the Poly Lens group, was casued by an CSV import, where I added a devices to the Poly Lens group. No matter if I choose replace or merge. So from my point of view it is a bug in the import service.
I would suggest you to create a Poly Support using you Poly Lens Tenant ID as ref. You can get the Tenant ID from the URL in your browser.
This will bring more focus on the bug.
Please remember to answer as "Accept as Solution" if you could use the help.