Missing soldermask on some pads of mcu
Notice that the images of your soldermask simply show large rectangles which span multiple pins. I'm assuming that this is accurate, although perhaps if you zoom in you would see that they are actually smaller rectangles which just look like they're connected.

Anyway, if they really are large rectangles, this instructs seeedStudio to remove these areas completely. That is, as submitted, none of the pins should have soldermask between them.

It looks like seeedStudio's algorithms added soldermask webbing for you where it seemed appropriate, but the algoritm failed to know your intention for the bridged pins.

I haven't used KiCAD, but either your footprint doesn't have individual mask openings, or the software is set to gang them all together (likely in the Design Rules).

Regardless, these can be soldered with care, so you don't necessarily need to scrap the existing boards...

Good luck :)