You have a better ear than you think! I tried it again (and actually tried all the notes this time...) and the F is definitely flat. It's particularly noticeable in the 5th octave (i.e. if you play C5 D5 E5 F5), but all the F's appear to have the issue. I verified (by ear) against Pianoteq set to equal temperament.
The other notes sound OK. The A and B might be a tiny bit flat too but my ear isn't good enough, and if they're flat they're nowhere near as far off as the F. OP, how are you specifying notes/pitches to the synthesizer?
The other notes sound OK. The A and B might be a tiny bit flat too but my ear isn't good enough, and if they're flat they're nowhere near as far off as the F. OP, how are you specifying notes/pitches to the synthesizer?