CloudConvert.com might as well be my fucking home page.
Converting FROM a format to a DIFFERENT FORMAT can cause loss. In this case from one compressed type to another.
You are saying that you use an extension to convert from WebP to PNG, right? PNG is a lossless file format. It’s compressed, but losslessly. Like zip is also lossless compression. You can remove information to make it more compressible and then it’s a lossy process, but that’s not because of PNG, but because of the specific workflow.