Acrylic get hazy marks from the heat of the laser and the mirrored acrylic is especially sensitive to it. (It’s been discussed a lot and you can search the forum for ways to minimize it. ) Acrylic sheets have a lot of tolerance, so some areas are slightly thicker than other areas and that can account for differences in stuff cutting through or not. Acrylic gets very hot when cutting and the smaller or more intricate the cut, the hotter it gets. So I’d guess the small pieces cut through because they got a bit hotter than the larger areas.
To see the PG setting just click that little arrow on the right.
Here’s one discussion about it.
A link to a bunch more
https://community.glowforge.com/search?q=mirrored%20acrylic%20