Cisco called back today (5/16) to follow-up and clarified they believe the issue is with an Amazon update to the Fire tablet.

I called Cisco Webex tech support and explained the issue on 5/11. They escalated it to tier 2 support, who indicated an update on the Fire tablet released 5/8 was to blame. It was unclear if this was a Webex update or an Amazon Fire update. Support ...

We also have a user joining our Webex from an Amazon Fire tablet that started experiencing this same issue.