Check whether a background is real image content or simply the way an app displays transparent pixels.
A viewer must show something behind transparency
Transparent pixels have no fixed visible background. A photo app may display them against black, white or a checkerboard. Seeing black in one viewer and white in another can therefore be normal. The important question is whether those pixels allow another layer to show through when the file is placed in a composition.
Perform a simple verification
Open the downloaded PNG in an editor and place it above a bright colored rectangle. Move the image slightly. If the empty areas reveal that rectangle, transparency is present. If a fixed white rectangle moves with the image, the file probably contains an opaque background. Confirm that you exported using Transparent rather than White or Custom color.
Also check whether you saved a screenshot or passed the file through an app that converted it to JPG. Both can flatten the original alpha channel. A .png filename alone is not proof of transparency; PNG can store fully opaque images too.
Return to the last good file
If an intermediate app flattened the image, reopen the original downloaded cutout and share it as a file where possible. Do not repeatedly erase white from a damaged copy when the transparent master is still available. Keep both the master and any intentionally flattened delivery versions clearly named, especially when someone else will receive them without seeing your editing process.