I don’t know if this works at all at the moment, but I’d say that is a bug. If you import an sRGB image/movie (and you correctly set the format to sRGB) blender should do a sRGB → linear transform before working with it internally.
If you then export everything to filmic ofcourse it will apply the linear->filmic view transform to everything, that’s what you tell it to do!
So I don’t think it’s
sRGB input → filmic transform → output
but rather
sRGB input → sRGBtoLinear → internal linear format → filmic transform → output
Now this is all ‘theoretically speaking’ because I don’t think blender works completely in linear space internally? I did see 8 bit/channel textures passed into cycles, but maybe cycles does the sRGB->linear mapping on the fly? I’m not really sure how this works internally. If someone has better knowledge please correct my assumptions!
Anyway, moral of the story is: if you have a background image/movie in sRGB and you want to overlay your new layers on the original image you should output your layers in sRGB. Or convert your background image to linear, do the merging, and convert everything back to sRGB.
If you import sRGB and set the output to sRGB the result should be identical (except maybe for some slight rounding).