Trouble shooting for an ultrasonic flow meter

in-line sensors or clamp-on sensors? Transit time or doppler technology?

What does "huge" mean? What's an example of normal vs "huge" flow?

Negative means reverse flow. Do you only see 'negative flow' when the system is not running and is supposed to be turned off?
It is a clamp-on sensor. transit time, for RO water. "Huge" means -99999L/min vs normal 20L/min. It only shows negative flow as soon as the system is started up.
For decades, clamp-on ultrasonic was the domain of several major players, but some break through in technology brought dozens of secondary players to the market in the early-mid 2000's. They all do it their own way. This sounds like one way of doing "fault" condition.

The negative -999999 value is very likely the "failure to obtain a valid reading" error code because it is so similar to upscale or downscale fail-safe mode read-out. The probability of a reverse slug flow at that velocity approaches zero, so the value is likely a failsafe value.

Siemens clamp-ons put a large F (Fault) in the display when signal analysis had low confidence in the signal value (too much suspended solids, too many bubbles, low signal strength due to poor transducer contact with the pipe, excessive scale/dirt, transducer misalignment).

Start-up might do something like draw a temporary vacuum in the pipe, where the vacuum can't carry the ultrasonic pings. Failure to receive the pings at the receive transducer then triggers the fault.

Transit time does not have a lot of tolerance for air bubbles, which will typically fault out transit time. I know, no one's system ever pumps air and their X-ray vision confirms it.