Scale Video, Without 'ffmpeg Scaler' converting colorspace from RGBA to YUV

I have a video in RGBA space contained in MPNG codec. Which is in 4K. I need to Convert that into a 1080p File… SO i Used this –

ffmpeg -i input.mkv -vcodec libx265 -vf scale=1920:1080 -sws_flags lanczos+accurate_rnd+full_chroma_inp+full_chroma_int -pix_fmt gbrp "x265GBRP.mkv"

But, I think ffmpeg converts rgb to yuv420p for scaling the
footage…. I can See some Artifacts that yuv420p creates, like when
you have Gradients… And also the Color is WASHED OUT! in output

This Image was the Output:

And this was the Input Frame:

NOTE: As both the Files are in RGB ColorFormat…. My Video Decoder is Not the Problem…. (And these Images are Screenshots of Extracted frames from the Input and Output File)

DEBUG LOG – txt File – GDrive

Something extra –