Notes land on the click
The song's notes sounded a few milliseconds after the metronome's click. They now start on it.
With the metronome on, each measure's first note came in just after the click, so the click seemed to rush ahead of the guitar.
What changed
Every part goes through a limiter on its way out, which keeps loud passages from clipping. To catch a peak before it arrives, a browser's limiter holds the sound back by 6 ms. The metronome doesn't go through it, so the notes always came out 6 ms after their click, and after a recording played along with the score too. The notes now start 6 ms early to make up for it, so they come out on the beat.