Versatile input termination / translation for reference clock?
Transient simulators typically start all capacitors with 0 V across them. That will put the buffer input initially at 0 V. You will need to run the simulation for a few times the time constant (100 nF * 50 ohms, or about 5 us) to get C1 charged up and put the buffer input voltage due to V2 near the steady state value.