My delonix regalia (flame tree) is losing its lower leaves. Any idea why?

[BonsaiQueen]

Hi, all. The lower leaves on my delonix are turning yellow in the center and then falling off. However, this is only happening to the leaves on the lower limbs. The leaves on top look very healthy and seem to be thriving. Right now I use rainwater to water mixed with bonsai pro fertilizer (7-9-5) mixing 1/4 tsp. Per gallon of water. Pictures posted below for your reference.

Can you all please share your ideas on what might be happening with my delonix. Is this normal? Indication of a problem?

 

[Ryleigh]

This is currently happening to my delonix regia plant that is about the same size as yours but definitely younger. I grew it from a seed and don't know what to do! Did you figure out the cause?? How did you fix it?

 

[BonsaiSniper]

is it on the lower foliage?