Adobe Rush playback of mp4 looks terrible

Hi there,

I've just rendered a short clip twice.  Once at 1080 and once at 2160.  Frame rate 30.  Shot on iphone 11.  When i play back the clip in Rush, it looks pretty good, mostly sharp.  When I play back the rendered mp4 on either my imac or my 27" samsung monitor, it looks terrible.  Here are 2 screenshots to show the difference between Rush playback view and the actual mp4.  Usually its the other way around with quality!  I saved to local drive so kept at high quality save.  Is this a bug?