Kodi only outputs progressive. In other words, 1080i does not exist, only 1080p. Kodi needs to render each interlaced half-frame as one real frame, so needs to use 1080p50 to render 1080i content. Do not whitelist 1080p25, do enable 1080p50 (and adjust refresh). Do not "sync playback to display" as that will force everything to be resampled and ARM devices are rubbish at that.
I initially set up my new RPi4B with the 24Oct20 LibreELEC release. I had judder and interlacing issues when playing back DVD Video_TS folder files and judder issues with NextPVR live TV and .ts files from NextPVR.
To fix the DVD Video_TS folder files issue I set "Sync playback to display". Video_TS folder playback is now smooth. I have to set "Sync playback to display".
Setting "Sync playback to display" did not fix live TV or .ts file judder issues, nor did it seem to make it worse. I determined that I only have judder issues on all 1080i stations. 720p stations work just fine. The other interesting thing is that I saved the .ts and played it with VLC after configuring the RPi4 for Raspbian, on which it played smoothly.
I am a newbie to RPi/Kodi/LibreELEC. It seems to me that the RPi4 is capable of smooth 1080i playback based on seeing that with VLC, but Kodi 18.7 does not seem equally capable. Or, maybe I don't know how to set the right settings differently from the defaults?