Hi guys, I am using Shape Match vellum constraint, but for some reason, it's not working as it is supposed to work. Instead of being solid and hard, it's melting. check the attach image please. Thank you

That's pretty much how shapematch works and also probably why it's not called rbd constraint or similar

It simply tries to match the rest shape with no regards to collisions and of course the collisions then try to resolve and pull some points back

To minimize this pull the best way is to make sure you use as low res proxy geo as possible
Then also increasing the collision iterations can help as they are interleaved into main iterations so having collisions update more offer can minimize distortion as constraints and collisions communicate more often

Then lastly increasing substeps may also help a bit

Thank you