I'm using LR Android v7.2.1 on a Samsung SM A520F, Android 8.0.0 including a creative cloud subscription.
Since a while the thumbnails in the library started to switch to black until today all previews have become black. I can edit the single images on my mobile and save changes and on my desktop LR works perfect. It's just the black previews. But this is rather restricting and bothersome!
Could you please share the type of images you are working with, JPEG or RAW? Have you recently imported a large number of images? Does your Phone have sufficient free storage space? Have you tried logging out and logging in again?
Before you uninstall Adobe Lightroom on your Phone, please log on to Lightroom Web (https://lightroom.adobe.com) to check current Sync issues on the left part of the screen.
If there are any sync issues, click on them to know more, find the original for those images, and back them up safely before you empty the Sync issues from Lightroom Web and then re-install Adobe Lightroom on your Phone.