Transparency turns white — read this first
JPEG has no alpha channel, so something has to happen to transparent pixels. This converter fills the canvas with #ffffff before drawing your image, which means a fully transparent pixel becomes pure white and a half-transparent one is blended towards white: a 50% red pixel over nothing measured rgb(255, 127, 126) in the encoder test. On a photograph that is irrelevant. On a cut-out logo it means a white rectangle behind the shape and a white halo along every anti-aliased edge the moment you place it on a coloured background.
The flattening is permanent. No later step can separate the subject from that white again without redoing the cut-out, so if the transparency is part of the picture, target PNG instead and keep the alpha channel. Convert to JPG only when you know the image will sit on white paper, on a white page, or inside a form that demands a photograph.