When an image is uploaded, it is resized to several registered sizes. Each size can have different settings for how to manage images whose aspect does not match the defined size. See https://developer.wordpress.org/reference/functions/add_image_size/
You can get those black bars when cropping is disallowed and the upload aspect does not match the defined size. This is only one of several possible explanations. Plugins and themes can alter how uploads are handled and can introduce their own quirks.
If you cannot tolerate cropping and cannot tolerate black bars, the only solution is to upload images that are already at the correct aspect.
