When a player is standing on a server-owned moving assembly (driven by AlignPosition), creating a WeldConstraint to add a new part and then destroying a solver-based constraint (AlignPosition, AlignOrientation, LinearVelocity) anywhere in that assembly in the same frame causes the player to lose platform standing/relative physics. The character slides backwards on the platform as if on a treadmill. The server sees no velocity change — the desync is purely client-side.
THE REPRO STATED ABOVE IS THE ONLY REPRO I COULD FIND THAT WORKS 100% OF THE TIME, MY PLAYERS HAVE ALSO REPORTED LOSING RELATIVE PHYSICS/PLATFORM STAND UNDER DIFFERENT CIRCUMSTANCES - THOUGH I HOPE THIS REPRO POINTS TO SOME LEADS
Reversing the order (destroying the constraint first, then welding) does not trigger the repro
This only occurs with StreamingEnabled disabled.
It is unknown to me how long this bug has existed in the engine
Simple baseplate file with repro (STAND ON THE MOVING PLATFORM AND SEND “p” IN CHAT TO TRIGGER THE REPRO):
AssemblyPhysicsBug.rbxl (67.2 KB)