Hitting a power ceiling could be happening on the vortex. You could test it by using the same gear and cadence when fresh, and see if you can get to a higher power.
It could also be an erg mode issue where it just stops following the demand power - I’ve had that in a ramp test. If you have erg mode issues with other workouts, that might have happened.
Third possibility, am I right in saying that your wife is quite new to this type of training? Maybe she wasn’t actually near her limits, and that produced the odd result.