MBP A1502(2015) SSD Link Width: x1 not x4

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Hi, I have a MBP a1502, the disk speed is 330/380

I check the system information
the Link Width is x1, I remember it's x4 before

so I replug the SSD and restart it yesterday
It's x4 now, and the disk speed is 620/1380

but today, when I check it. the Link Width is back to x1 again
disk speed back to 350/350 too
what is the problem?