UE5 uses some AutoScreenPercentage,
you can try changing it in your Project Settings:
Project Settings > Editor - Performance > Viewport Resolution,
change it to Manual and adjust your ScreenPercentage and Minimum Resolution.
Or try these values, they worked on my Project:
r.Editor.Viewport.OverridePIEScreenPercentage 0
after that you can use r.Screenpercentage again in your Playtest.
Hope this somehow helps.