RFC483 say:

 Prefix          : A constant 28-bit-long bitstring value
                   (2001:10::/28).

RFC483 check is perform there:

https://github.com/bitcoin/bitcoin/blob/master/src/netbase.cpp#L403

We check:
GetByte(12) & 0xF0 == 0x10

But we must check:
(GetByte(12) & 0xF0) == 0x10

it seems to be typo