Is there image degradation when you apply the same chroma subsampling iteratively?
If the target chroma subsampling is the same as the source subsampling, then any ffmpeg scaling will be a no-op in that regard, so it's not subsampling 'further'.

FFmpeg does not determine chroma sampling scheme from a tag. It is a basic property of the decoded bitstream. If it's 4:2:0, then the decoder will emit chroma planes of half-width and height with respect to the luma plane; for 4:2:2, half-width and full-height. A scaling operation will be carried out if the input and output schemes are different. A scaler will resize each row of the chroma plane to the target width of the output chroma plane and then rescale the resized rows to obtain the target height of the chroma planes. The "translation" is done as per the choice of scaling algorithm - see list at https://ffmpeg.org/ffmpeg-scaler.html#Scaler-Options