Multiresolution creates strange geometry

Hello, I think I’m facing a bug with the multires modifier.

I applied the multiresolution modifier on a mesh, and I had started sculpting it. I extruded a part of the low-poly mesh, but the modifier gives a very strange result. I tried a few things to minimize the damage, but these faces are never correct:

The normals are seemingly not the problem. Removing the multires and adding a new one works, but it deletes all the sculpting I had done until now!
Could someone help me? multires_problem.blend (766.9 KB)

check for double edges, use merge by distance…

Thank you for your answer, but I don’t think my mesh is the problem.
Here, I deleted all the faces with weird geometry. But after I extrude like I would normally do, the problem is back.

check in the interior if there are any edges or tiny faces there…

are you using 2.9 or 2.91? 2.91 can be a little unstable…

Hello, I am using 2.83.
I have examined this mesh in all the way I know, and I haven’t found anything special about it.

I should be more specific about the occurence of the bug:
At first the whole top of the mesh was anormal, I separated it to another mesh, then joined the two again and only those faces that you see have the problem now.
I didn’t edit the top, just made some operations with the modifiers and it became okay. I think the bug doesn’t come from the edges or faces…

Have you tried downloading the blend file, and do you have the same result as I do?

ok remove your multires modifier and re add it, you sculpted in the 2-3 sub-div level and createdan little artifact, i opened the file in 2.79 and removing and adding it again worked…

Hello, thanks for the help.
The project I sent was only a part of the mesh, the one with the problem. It’s a part of a body I was trying to make.
I’ve “erased” the weird shapes with the sculpt mode, and it works in this case. I can keep the other sculpting I made. removing the modifier and adding it again worked in 2.8 but makes you lose all the sculpting.
Nevertheless the problem is worrying, it should never happen in the first place. I don’t know how, but maybe it should be reported to the devs.

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i dont think is a bug, but report it in case, use the option report bug in blender, in the help tab. Because you sculpted in a lower level (like in the 1 or 2), thus you created some bulges, or maybe was something else from the modifier, because the mesh was not having a issues, but the modifier had.

Not sure what you’re talking about, you must have somehow sculpted what we see, if you use the Smooth brush (select any brush and press Shift) you can make this cavities disappear

Im agree with him.
I think the same

I’m absolutely sure I never sculpted those shapes. Here is video proof the modifier is acting on its own:
multiresolution.rar (527.9 KB)
You can try by yourself in the project I sent.

it must have to do with how multires assigns informations for each vertex, a bit too technical for me :confused: