When "Kinetic buff" work your mag is big, and you can reload then you lose all the additional bullets.
But when Fire rate buff ended you can't reload, and you can just holded all additional ammo. View attachment Вырезать_1 752 818 921 371.mp4
It's kinda irrational because item discription tell us, 7 sec buff +30%FR and +8 ammo
All heroes that have an alt fire (Shiv, Viscous, Yamato) can reload immediately after gaining the buff, even at full ammo, causing them to lose the fire rate bonus instantly, but keep the extra ammo.