Poor transformer voltage *after* installing power kit v3

Hello, I have a 16VAC-30VA transformer (actually puts out about 18VAC) that has been successfully powering two Doorbell Pro 2s that were directly wired to the transformer, no power kits. Today I wanted to hookup a wireless transmitter for the wireless doorbell chimes in the house. Once I wired up the power kits both doorbells complain about poor transformer voltage. In the doorbell settings they both say 15V. I also confirmed that I have the same problem with only one doorbell and power kit, so it's definitely the power kit dropping the voltage. Curious about a couple things: 1.) Why does the power kit drop the voltage? (That seems kinda ironic but I don't quite understand how it works.) 2.) What's the best solution? Two 16V transformers, or a bigger transformer? (I was going to get a 24V, but all the reviews complained about heat and buzzing noises, the 16V has no issues. Any 24V recommendations?) (In the photo white wires go to transformer, red go to doorbells. Terminal block is for convenience while testing.)