When you export a structured workout to Zwift, by default Zwift does not show the average power for the current interval or lap, just the instantaneous (1 second power).

This makes it hard to really hit your target on longer intervals and is easily solved by instructing Zwift to show the average power for the current lap by adding the show_avg="1" attribute to the <SteadyState> block in the exported .ZWO file.

Here's some documentation on the feature: https://github.com/h4l/zwift-workout-file-reference/blob/master/zwift_workout_file_tag_reference.md#attribute-show_avg

And here's what it looks like in Zwift: