Video files not played/freezed when forcing 1080i50 - Raspberry Pi 4

Hi there! 

I’m facing an endless problem with my Raspberry Pi 4 (1GB) (I also have a Raspi 3 B+). 
Both of them are “forced” to work in 1080i50 because the right CEA mode 20 was not available under raspi-config (no interlaced mode was available). 
Both of the raspi are connected to a video switcher (working on 1080i50) via HDMI ports; both of them with the same OS version (Buster, last update).

The problem is that, on the Raspi 4, when i try to play a video file (.mp4, .flv, RTMP, 720p, 1080p, 1080i, everything) with omxplayer or ffplay, the video freezes on the first frame until i stop the process (ctrl+c). The only way to achieve a different result was playing a 1080i50 RTMP stream (wich i was able to see clean on other devices): in this case i have a video that plays at something like 1fps.

With the same setup, and same video files and players, the Raspi 3 B+ works really good without any problem. 

On te Raspi 4 i tried to: disable GL, disable 4K60 and any other advice i found on this forum and also on github etc. Nothing worked for me. 

At this point i tried to clone the SD card (same model, same size) from Raspi 3 B+ to Raspi4: everything worked good. But the video player in HDMI forced mode does not work. Same problems. 

Does anyone know this issue? I have no problems on Raspi 4 when i set a resolution from the raspi-config , and every video file is played properly, but i need that the raspi MUST be on 1080i50. Why the same thing, same os, same config, same everything, works differently on Raspi 4?

Thanks for any kind of help!  :)