Issues about driving a high current DC Motor using an H-bridge
your circuit seems fine , but it doesn't contain a current limiting protection . and you might be driving the motor directly without ramping the PWM . So my guess what is happening is that the motor is accelerating and decelerating quickly causing High voltage spikes on the 24v bus . by the way if your 24v is from a bench power supply then it can cause a disaster , it should be battery or batteries in parallel to power supply to absorb the regeneration energy .

Another aspect is the DC DC converter choice , 1W might not be enough and isolation is not needed since you are using a bootstrap driver . So use a 7812 instead it will work flawlessly.

So my proposed solution : 

1. Use Battery for 24v supply 

2. use a TVS on 24v bus to absorb the transients 

3. Ramp the PWM every 1 ms 1% for example , to accelerate/decelerate slowly to prevent spikes from happening 

4. Use a small 1A to 2A dc motor to test the system , check the signals with oscilloscope , if every thing works fine , install the big motor . 

general note: NEVER put the oscilloscope ground pin on the Vs terminal when trying to measure the high side transistor gate . instead measure the High side gate signal with respect to ground ( it will be around 36v).