fix over exposure in importance sampling in screen space reflections
After some testing and some research I have found that both the $\sin(\theta)$ and the $\cos(\theta)$ should be removed from the denominator.

the reasoning is that, since we want a probability function of steradians instead of angles, the $\sin(\theta)$ factor (which is essentially used to convert from steradians on a unit sphere to radians on the two polar axis) should be dropped.

Also, $\cos(\theta)$ (as Nathan Reed explains in this blog post) is there to convert from microfacet area to macrosurface area. Since we are generating microsurface samples, this should be removed.

After these corrections we get the formula

$$L_o(p,\omega_o)\approx\frac{1}{N}\sum_{i=1}^{N}\frac{L_i(p,\omega_i)FG}{4(n\cdot\omega_o)}$$

which is free of the artifacts of the formula on the question.