Delonix regalia losing lower leaves?

[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?