AmigaOS does indeed enable the extra half line when any visible viewports are interlaced. Fortunately that just works because the individual “non-interlaced” fields are still legible when shifted back and forth by a half line.
Forgot to add this earlier. Later versions of AmigaOS that support non-15kHz rates will promote all screens to the lowest screen mode that can suppprt all visible screen resolutions. So, if you drag a NTSC:High Res Interlaced (640x400@60i) screen down in front of a Multiscan:Productivity (640x400@70p) screen, then the front screen will also display in Prpductivity mode.