The third and lowest diode makes the data out line from the 4021 input shift register be effectively "open drain". You cannot tie a pushpull output from a chip right to the SDAT line.

I missed that the chips were 40xx, That does not make sense. IIC driving 40xx ?. If you were to drive 40xx (or any other standard shift register), why not use straight data and clock similar to SPI?.

Erik