The M.2 sockets on motherboards don't exactly have great cooling, and SSDs don't usually even come with radiator.
The positioning can also be pretty iffy, mine have one next to CPU, another just under GPU (no chance getting fan there) and those are the "fast" (directly connected to CPU) ones!
The another 2 slots are again under GPU (one filled with wifi/bt card), and only last 2 are far away from other hot components and get its own heatsink. but those are not directly connected to CPU