My refrigerator is like your water heater. Sense detected it relatively early, then lost the detection and support told me to delete it. It has not come back, even after waiting long enough. I suspect that the reason is in the quote above. As you note, a full reset seems overkill for getting a single device to reappear. I know you said they told you they can’t help with device detection, but maybe they didn’t realize that you are asking for a redetection.
It might be possible for technical support to go into the account and remove the deleted detection, which might allow detection to occur naturally. @JustinAtSense, do you know if your team can do this?