The seed has 132 bits and the length of the prefix is 8 bits. Therefore, 8 bits are "lost" by imposing the 0x01 prefix.

However, there is no way to enumerate seeds that hash with the desired prefix, other than brute force. Therefore, from a security point of view, these bits are not "lost"; an attacker still needs to use brute force in order to find valid seeds, just like they need to use brute force in order to attack the remaining bits.

There is still a weakening of the seed that results from the imposed prefix, because no key stretching is required in order to generate the prefix.
But it is wrong to express it in terms of "bits lost"; all you can say is that these 8 bits are easier to enumerate than the remaining 124 bits.