My LO started out at 25th at birth and then went to 5th for most of her first year. She was 50/50 bm/ff. I worked and didn't produce enough when I pumped to be ebf.
I was really worried most of the time in the early months. I weighed her obsessively. My ped (40 yrs of exp) told me not to worry as long as she was happy and meeting other milestones. At 2.5 yo, my girl fits in 18 mo clothes, is in 8th percentile, and runs around like a crazed monkey healthy as can be.
So my response is two fold. 1) supplementing w formula may not magically increase weight. If your LO is destined to be in the 5th percentile, that's where they'll be. 2) don't be too worried if the docs not worried.