It’s not really a bug, so I assume this is the right place to talk about this.
I was studying the EEVEE bloom effect because it is gorgeous, but it seems that there is a divergence with the KinoBloom implementation it is based on (which is probably based on the method explained here).
In Blender, it will bind the source at
source/blender/draw/engines/eevee/eevee_bloom.c:326 and blur at
source/blender/draw/engines/eevee/shaders/effect_bloom_frag.glsl:190. If I’m not mistaken, it is adding the upsampled data with the blured previously downsampled buffer at the same step (the same size, not an upsampling).
Is it intended ? I wonder if it looks better in any case.
Also, by doing the blur in chain, we only need one texture of each size for the downsampling and upsampling phases (downsample blur to N-1 texture -> N texture -> upsample blur to N-1 texture with additive blending).