I believe Larry cad has the magic answer.
I experienced similar symptoms as you. Replaced every electrical part of my water heater and followed the Atwood troubleshooting guide multiple times. After installing the third New circuit board I also spent extra time making sure the ground was connected, metal to metal with a good tight screw.
Now everything works as designed.
I tested the continuity of the ground numerous times and it always tested good so kept looking for something else. But in the end I think it was the ground.
Here is a good resource for information.
https://techsupport.pdxrvwholesale.com/
Good luck.