Real 2FA would (theoretically) never make your account less secure than 1FA, because even if the second factor has 0 security, it shouldn't decrease the security of the first factor.

However, it is true that this may not always be the case for imperfect implementations, like your example. I can aldo imagine that social engineering might have a higher succes ratio if the intruder can say "it really is me! I have the correct second factor, I just lost my first factor...".