Failure messages while testing Sonar

Why do these messages suddenly show up while testing Sonar on different frequencies:
MONO_18 18000 - 18000 failure
MONO_19 19000 - 19000 failure
MONO_20 20000 - 20000 failure
CHIRP_18_20 18000 - 20000 failure

This happened only once.
After I closed the app and opened it again, tested the Sonar again on all these different frequencies, it showed the message “Sonar is supported”. So it seems to be ok.