After exporting from Lightroom, the quality of the photo differs.

Hello. I recently reinstalled Windows. Now in Lightroom, in the "Develop" section, I see a good quality photo. In the "Library" section and after exporting to jpg, the quality is worse, "squares" appear in the shadows. Photoshop opens these files just as badly. Moreover, my phone shows the same jpg files well. Maybe it's not about Lightroom? Thank you.