Hi @Charles,
Most likely the altitude distance was too short for the sonar to detect with its configured scanning range and gain, in which case the next strongest response is likely after the ping has reflected up off the bottom, down off the surface, and back up off the bottom again:
\begin{align}
\text{measurement} &= 2\cdot\text{altitude} + \text{depth}\\
\rightarrow \text{altitude}_\text{corrected} &= \frac{\text{measurement} - \text{depth}}{2}
\end{align}
The ArduPilot ping library doesn’t seem to do any configuration of the settings, so the sonar will be operating in its default automatic mode.
I would expect the profile data to show usable signal at the range you were at, but its possible the distance estimator could be struggling there and start searching further away, especially if there are weak reflections from things like plants, fish, and particulate matter between the bottom and the sonar.