I have experienced the same problem. When Faolain first instructs Caithe to knockout the guard and I am given the option to interact, though I sit jamming away at the “f” key, nothing occurs.
That is to say, the interaction does not occur. In fact, Caithe readies her weapons, and my skills lock out as though I were engaged in combat, but the channel for the knockout never happens.
If necessary, I would be willing to attempt to capture footage of the bug in question. Thanks for your time and trouble!