What if an attacker knows 1 bit of a OTP ciphertext
No, this wouldn't cause problems.
The reason for this is the way how perfect secrecy is defined.
The definition says that a scheme is perfectly secret if an attacker doesn't learn anything new about the message when giving a cipher text, also see my other answer for a more "mathy" formulation.

So an attacker knows a certain bit is fixed, thus he can't ever learn anything new about it from the cipher text. And he also can't learn anything new about the rest of the message because only the corresponding key bit is fixed and the rest of the key stream is still uniformly and independently random and is thus still an OTP.