Bumping thread because of the same issue, i decided to anchor everything when I weld it in the build area, but weird behaviour still presists when unanchored.
Parts show up with normals inversed on the server
EDIT: I checked and confirmed that the parts are being placed in the right position
EDIT 2: Code sample, C1 = p1.CFrame:toObjectSpace(p0.CFrame) didn’t work for me
local weld = Instance.new("ManualWeld", base.Welds)
local CJ = CFrame.new(m0.PrimaryPart.Position)
weld.C0 = m0.PrimaryPart.CFrame:inverse()*CJ
weld.C1 = m1.PrimaryPart.CFrame:inverse()*CJ
weld.Part0 = m0.PrimaryPart
weld.Part1 = m1.PrimaryPart
Edit 3: Here’s what happens with weld constraints, but some odd velocity gets added
