Q.Why my gardenia buds are falling off before they bloom
It is a tree in a pot; did replant in bigger pot a few weeks after buying it; had some yellow leaves on bottom branches near main stem
Gardenia can be very finicky and the transplant may be the reason for the bud drop.
These articles will refresh you on the care.
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-flowers-gardenia-buds-falling-off-plant.htm
https://www.gardeningknowhow.com/edible/helping-a-gardenia-bush-with-yellow-leaves.htm
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm