Lightroom preview stuck in low-resolution for all photos

While processing some photos the other day, the previews for all of them became pixelated and low-resolution. I'm unable to see the sharpness and focus of any image when I zoom in. I've noticed in the past that images last worked on at least a week ago become like this, but starting to edit them causes them to return to their true look/resolution. However, Lightroom has been stuck on this low-res preview no matter what I do. I can only see the true preview when using the Enhance/denoise feature. I've attached two images to show off this issue.

I've read that changes to image previews are related to performance, so I've tried a variety of fixes to try and resolve this issue: setting Lightroom to High Performance graphics in the Preferences menu, clearing out space on my computer, and running Support Assist to boost performance. None of these have helped. I recently hit 19,000 images in my Lightroom library, so perhaps this high volume of files in the library is the issue? If not, does anyone know why this preview issue has arisen and how I can resolve it?