That is really odd and really makes me wonder if something is wrong with your hardware. The one test I’d like to try, if you’re up to it, is to disconnect and reconnect the power to the IMU2 after the ROV has already booted up and you’ve received the BNO_INIT_FAILED message. The code for the IMU is designed to try and reinitialize the device every 10 seconds or so, and there have been problems in the past where it could get in a bad state and needed to be entirely power cycled.
Doing this is easy, you just need to clip the positive power wire on the IMU, preferably close to where you made your solder connection so that you can easily repair it afterwards. Once it is clipped, strip a bit of coating off of the ends and then use a breadboard, alligator clips, or whatever to connect them back together again, watching to see what happens in the cockpit message log for the next minute or so. If the IMU doesn’t successfully initialize after doing this, try rebooting the ROV with the IMU completely disconnected at the time of boot and then connect it once you can load the cockpit again, again watching the output window to see if it successfully initializes.
Let me know what you discover!