What the conversion does to the picture
The WebP is decoded and re-encoded as a JPEG at high quality, which is a second lossy step on top of whatever the WebP already did.
Both formats are lossy, so the result passes through compression twice. At the quality level used here that is normally invisible on a photograph, but it is a real and permanent change, and it compounds if you keep converting the same picture back and forth. Convert once from the best source you have, and keep that source. Where the image is a logo, a screenshot or anything with hard edges, the second pass is more likely to show as faint halos around lines.