We've recently made an application using Vuforia that we're planning to publish shortly, but we've run into a bit of a bug. We noticed recently that when we use any version of Ar Core, from the recommended Ar Core 1.4 library upwards to 1.10 (tested with 1.4, 1.6, and 1.10 libraries) autofocus does not work on several phones supporting Ar Core (Galaxy S8 and Galaxy S10 tested). After looking into the issue on the forums I decided to remove the library as the app did not use positional tracking. This solved the issue on the Galaxy S8 but disabled the camera altogether on the Galaxy S10, trying to view an ImageTarget results in a black screen (specifically the camera render surface, the UI being drawn in front of it was still rendered and responsive).
I attempted to upgrade to the newest version of Vuforia to see if it would resolve the issue, going from Vuforia version 8.1 with Unity 2018.3 to version 8.3 with Unity 2019.2 (doing so did break a significant portion of the app that was unrelated to Image Tracking and would be a significant extra time cost to development) and the app, deployed to S10, rendered a black screen even when including the Ar Core 1.10 library which rendered video without autofocus in the previous version in the same case.
I'm using the continuous focus feature which works well on iOS and all other Android devices we've tested with (S6, S7, S8, Generic Tablet).