Gate Driver Circuit Gives Unexpected Results!
The upper driver inside the chip that drives the top FET gate is powered by the bootstrap cap C29. Since this only stores enough energy to power the driver for a short amount of time, this kind of driver can't keep the top FET ON continuously, it has to pulse the bottom FET ON to recharge the bootstrap cap.
When trying to keep the top FET ON continuously by holding pin 1 to logic 1, what you're seeing is expected. First the top FET will turn ON, then after a delay the bootstrap cap will run out of charge and the top FET will turn off. On some drivers it turns off slowly, on others it turns off abruptly.

Measuring the HO pin, I see a spike of about 40V and then quickly goes to ~22V and then exponentially decays to nearly zero, and repeats all over again.

When output voltage decays to zero, the bootstrap cap recharges via the diode, so it is again able to turn on the top FET, and the cycle starts again.
R43 would discharge the bootstrap cap quicker, so it should be removed.