My son was around 7.5kg at 7 months, but honestly every baby is so different depending on their birth weight and genetics lah. The percentile drop sounds like it's what's worrying your PD more than the actual weight itself, so it might be good to dig a bit deeper with them on whether she needs more solids or if there's something else going on like tongue tie or latch issues even at this stage. Since she's drinking normally, it could just be that her growth is naturally slowing down or she needs more frequent solid feeds to match her hunger, but definitely worth getting it checked out properly rather than second-guessing at home.