When you're using a ringing filter, sRGB is often a better colorspace for upscaling than linear RGB, because it distorts the ringing artifacts to make them less noticeable by putting them mostly in the highlights, not the shadows. It's not mathematically "correct", but it's better suited to human perception, similar to how minimum phase filters can perceptually outperform linear phase in audio processing, where pre-ringing is more noticeable than post-ringing.

See "Catch #3" here:

https://entropymine.com/imageworsener/gamma/

And you can sometimes get even better results using a sigmoidized colorspace. See:

https://legacy.imagemagick.org/Usage/filter/nicolas/#detaile...