During my rides Swift does not seem to register any point where the power goes over 400w. When viewing the power graph on Strava there is a break in the line at the 400w point. I do actually think I’m going over 400w as cadence and speed are increasing when this happens!
There is a 400 watt upper limit to not controlled trainers (without power meters). That is, essentially using zPower as measurement.
My guess would be that you either have a “dumb”, not controlled trainer, or you have your trainer set up like that.