High Pass filter gain at higher frequency than cutoff frequency
The basic problem is you are getting hung up on irrelevant details.

The filter rolloff frequency is 10.3 Hz, and your input frequency is 300 Hz.  That means the input frequency is 29x the rolloff, or 4.9 octaves.  That's "a lot", so the output amplitude is going to be very close to the input amplitude.  Put another way, the filter gain will be 1.0 for practical purposes.

The expected filter gain is 1.0, and you're getting 1.00.  What's the problem?  You're upset because a numerical simulation is off by a fraction of a percent?  Really!?  Are you really going to implement this filter with a .1% resistor and capacitor?