Also found that some modifiers cause this behavior as well, such as displace

Would this bug fall under any of these categories? https://developer.blender.org/T78479

Brushes like grab will allow you to stretch the mesh as much as you want before it locks it, but brushes like snake hook which are constantly recalculating will stop mid-stroke. This means that some brushes would only have to recalculate after the stroke. I am not sure why the render mode affects it though.

Quoted Text

Trying to find a consistent pattern to when this happens is proving to be difficult. Sometimes, a rough specular highlight has no ring and looks fine. Other times, the highlight seems to disappear entirely, especially when the light is more distant. It looks a lot worse on metallic objects but increase the light's specular value makes the problem noticeable even on non metallic objects. Sometimes, it appears as a clipped highlight rather than a ring. It happens very randomly and I am not sure why.

Correct me if I'm wrong, but I believe that the reflection cubemaps and irradiance volumes use the same underlying bake method. I suspect this because the SSS behaves exactly the same when it comes to reflection cubemaps (light bounces ignored with subsurface). In that case the problem is even worse and more noticeable, especially when the material is perfectly glossy (the material with subsurface is just black). If so, I think that if the baking would treat the material as simply diffuse just like reflection planes do that would be perfect.

In the volumetrics tab on the right, you must have volumetric lighting enabled.

Can confirm the bug still occurs in the latest builds (2.91 beta and 2.92 alpha)

It never actually worked. There was just a version where I couldn't use multiple time steps due to a different bug (https://developer.blender.org/T79646). At first I didn't realize there was a bug, and no time steps meant no blending issues (since there were no steps to blend between) so I thought it was gone. The bug later got fixed (not this blending bug but the time step one) and the blending issues came back. The hash mentioned in the post is rBa93be410c974. So, even in that version the bug was not fixed. As far as I know, it has been there ever since the new motion blur was added. I hope I am clear.

Reproducible in newer versions of Blender

After I made the comment about the time step blur I noticed the other motion blur issues had already been reported.

Yeah, not having step based motion blur would kind of defeat the point of the new improved motion blur in 2.90. Let's hope that we get step blur back! 😁

About 1: It should be an option. If a user is only using one sample and there are post process blur artifacts then they might be confused as to why there are artifacts, while controlling the steps from the motion blur panel makes it intuitive that it should change the quality of the motion blur. Global samples shouldn't change the quality of a given effect, it should simply average randomized samples of the render. Effects like anti aliasing don't "care" what the global samples are. They simply randomize each sample and it will eventually converge. On the other hand, the motion blur would have to take the global samples into account for each sample so there would be accurate post process blur. Having it use global samples would be more practical but much less intuitive.

I tried with the experimental builds of 2.91, and the problem didn't seem to be there at first. However, turning max blur to 0 with a non-zero step value caused motion blur to not have any effect whereas in 2.90 there would still be multiple steps, just not blended using post-process blur. This leads me to assume that time step blur doesn't work in 2.91.

In T78479#970880, @Germano Cavalcante (mano-wii) wrote:

I can't replicate this part:

Try to sculpt the deformed area and notice it is unsculptable

Is there a step missing?

Yes, I forgot to mention you must be in rendered mode. Updated now

In T77658#949943, @Jeran (Thejeran) wrote:

Turn SSR off for perfect reflection plane. For some reason when they work together it causes glitchiness. Especially when change changing perspective.

One thing I notice is that the missing pieces of the reflection jitter wildly as the view changes. F12 renders are still getting these issues at certain viewpoints.