How to calculate MIS weight more correctly in pbrt-v4?

According to the book pbr-book chapter 16.3 multiple important sampling, for given t and s, we can calculate the weight among different connecting strategies according to pdfFwd and pdfRev. Yet when I render the scene with specular BSDF components, MIS seems to yield suboptimal results, which is not consistent with my UDPT and the BDPT rendering of mitsuba0.6:

My renderer (unidirectional)	pbrt-v4 (256 spp)

We can see that, pbrt-v4 here has some shot-noise. I didn't think it was related to low spp, so I took sometime to figure out what happened (because the same issue happened in my BDPT renderer, which is implemented according to the theory of pbr-book, and therefore, same issue can be observed in pbrt-v3 as well), I found that it was caused by SDS path (s = 0, t > 4), see the figure below:

260c6cdee9eaf3d6432de5e23a8526c5

For the blue path (SDS caustics path?) here, since the vertices in the path are alternating between specular delta and non-specular type, the logic of the following code will never add any ri onto the sumRi, therefore the weight 1 / (sumRi + 1) will always be 1., which is usually larger than the weight of the red path (since we have multiple connecting strategies, therefore sumRi is always greater than 0.).

for (int i = t - 1; i > 0; --i) {
    ri *= remap0(cameraVertices[i].pdfRev) /
          remap0(cameraVertices[i].pdfFwd);
    if (!cameraVertices[i].delta && !cameraVertices[i - 1].delta)
        sumRi += ri;
}

I think this doesn't make any sense, about BDPT being worse than UDPT. MIS weight is the weight of a strategy w.r.t all the connecting strategies given s & t, so... could it be possible that we've only focused on the relative value but have ignored the absolute contribution of a path? The weight w.r.t all other strategies (in the example I present, there is NO other strategies) is 1 but maybe it doesn't mean that its absolute weight is that big. I know that using MLT might help but... what about the BDPT itself?

The scene file can be found here: mirror-ball.pbrt. Also, if we set the material of the ball to be glass with delta specular reflection and refraction, similar issue can be observed.

mmp / pbrt-v4

How to calculate MIS weight more correctly in pbrt-v4? #326