When uploading images to the general media library, no alt text is auto-generated, though title text IS (complete with dashes from the modified file name)...
A title isn't required for validation... but an alt tag IS. That text should be put in both places or the alt field rather than just the title field...
Is this a known bug?
It's not a bug. The alt attribute should never contain the image filename unless you are referencing, for example, an image file download. Added to which, some images may be considered purely decorative - with no informational value. In this situation, it is perfectly correct (and possibly even good practice) to use a null ("") alt text.
But it doesn't add null alt text. If you validate, you'll have an error for every image where you do not MANUALLY add alt text. Hence a bug, imho.
I'd be happy with a null alt text inserted.