Since as far back as I can remember when using range, arrows will only stack up to 25 on the ground. Arrows used after 25 will appear as single drops instead of being in one pile. In RS back in the day if I remember correctly, arrows should continue to stack in one pile above 25 arrows.
The workaround to this problem now is picking up the 25 arrow pile and dropping them to have all of the arrows stack into one pile.