I have seen this issue on three devices running Android 10 with a native (not the universal) TeamViewer Add-On:
When connecting with the viewer, remotely I see a black screen with a white lock icon. I cannot see the actual Android lock screen, yet the device itself (when having it in my hands) displays the regular Android lock screen.
In other words, what I see through the remote viewer is different from what the end user sees on the device directly. And because the viewer essentially prevents me from seeing the Android lock screen, I cannot enter the lock pattern either. The local user then has to enter the pattern, and once the device is unlocked, I am then able to see the normal Android screen through the viewer.
I have seen this issue on three devices so far -- one Nokia 5.3 and two Lenovo Yoga Smart Tabs (YT-X705F) that were recently upgraded from Android 9 to 10, which is when the problem started.
I assume this is a security mechanism in some Android versions that purposely prevents remote viewers from seeing the lock screen, requiring the local user to unlock it instead.
The only workaround so far has been to turn off the Android lock screen security completely. But I find that to be a sub-optimal solution.
