Severe artifacts from Compute based Downsample/Upsample

Hello,

I'm currently trying out a very simple bloom solution using compute.
The idea is to recursively downsample the rendered HDR texture until this condition is met "(width ≤ 4 || height ≤ 4)".
and then start upsampling the downsampled textures in the reversed order.
Both shaders are extremely simple and really just takes a few samples around the current pixel and adds them.
The upsample also lerps between the current value and the upsampled value using a “strength” setting as factor.

Now the issue is that I'm getting severe flickering artifacts, in kind of a “tiled” looking pattern.
I've …