2015 MacBooks are not Display P3. What browser are you using, as that is incorrect behaviour.
The clipboard here is probably copying the image buffer in the untransformed state, and pasting it into MSPaint would dump those values incorrectly.
The clipboard here is probably copying the image buffer in the untransformed state, and pasting it into MSPaint would dump those values incorrectly.
The issue would be the mechanics of the copy (attaching metadata) and pasting (interpreting the metadata in conjunction with the source image buffer.)
So technically, no, there's no conversion on the source data, it's the representation in the pasted context that's the issue.