A transparent PNG can look perfect in a design tool and then suddenly show a white rectangle after conversion. This usually is not a bug. It is a format limitation. PNG can store an alpha channel, which means each pixel can be fully opaque, fully transparent or somewhere in between. JPG cannot store that channel at all.
When you convert PNG to JPG, the converter has to decide what color should replace transparent pixels. Most tools choose white because it is the safest default for documents, product listings and websites with white backgrounds. If your logo is meant to sit on a dark page, that white fill becomes obvious immediately.
Know which formats support alpha
PNG and WebP support transparency. APNG also supports transparency for animated PNG-style graphics. JPG and BMP do not support transparency in normal web workflows. ICO can support transparency, but it is meant for icons, not general images. If transparency matters, stay in PNG or WebP.
The confusing part is that some image viewers display a checkerboard background to represent transparency, while others display white, black or the page background. A file can still be transparent even if your viewer shows white. The reliable test is to place it on a colored background or inspect it in an editor that shows an alpha checkerboard.
Choose the output based on the background
If the final image will always appear on a white page, converting to JPG can be fine and will often reduce file size. If the image might be used on different backgrounds, keep PNG or WebP. For logos, stickers, UI icons and cutout product photos, preserving transparency is usually more important than squeezing out every kilobyte.
When you do need a JPG, create the background intentionally. Put the transparent PNG on the exact color it will use in the final design, then export. That gives you a deliberate result instead of a surprise white box.