Changing the size of welded Part1 causes weird collision on players standing on Part0

[homermafia1]

Reproduction Steps

• Create a part as the Part0 of the weld
• Create a part as the Part1 of the weld
• Weld them together (either using Weld or WeldConstraint, both cause the issue)
• Have a player standing on Part0
• Repeatedly change the size of Part1 (TweenService, while wait() do, etc…)

If there is only one welded part to Part0, the collision looks normal, but for 10+ welded parts, the bug occurs 100% of the times.
It happens with both R6 and R15.

Minimal repro (script in StarterPlayerScripts):
R6:
CollisionIssueR6.rbxl (22.4 KB)
R15:
CollisionIssueR15.rbxl (22.4 KB)

Expected Behavior
Normally, players standing on Part0 should not be affected by Part1’s size changing.

Actual Behavior
Any player standing on Part0 will have their legs go under the part and move very strangely.

Video of the issue:
R6:

R15:

Issue Area: Engine
Issue Type: Other
Impact: High
Frequency: Constantly

[Conejin_Alt (DisplayNameHere)]

I hope this is related, I’ve been experiencing the same issue around a week ago with my building system which uses welds, I would like to point out that in my case:

• If I weld X part to Part0 while standing on Y part which is already welded to Part0, the bug still occurs.
• This bug is more intense when I start to spam parts.

Here’s a clip of this bug occurring in my building system:

Here’s another clip, but this time, when I spawn a part its size increases like OP’s video:

Demo Place: weldBugRepro.rbxl (38.4 KB)

I’ve seen this bug in other building games like Build and Survive and Creeper Chaos (here’s a clip I found where you can see this bug occur, since Creeper Chaos is no longer available)

I had a temporary workaround before which would be welding the parts to a clone of Part0 that doesn’t have collision, but I doubt that’s something optimal.

[BusyCityGuy]

We see this same bug in Eclipsis. The terrain is many blocks welded together, and the crystals are welded to the terrain. Building a new structure on the terrain also causes it to happen, as the structure gets welded to the terrain. These are some videos from our players:

We first started seeing this reports of this bug from our players at the end of March of this year. I haven’t been able to reproduce it in studio, but it happens near 100% of the time in live matches.

[gigagiele]

Thanks for the report! We’ve filed a ticket to our internal database and we’ll follow up when we have an update for you.

[Hajimalago]

Hey! Sorry for the late response on this issue. We identified the issue and it should be addressed now. Let us know if you run into any more issues!

[chillthrill709]

Build a Boat for Treasure experiences this issue too. More so related to @Conejin_Alt 's post where he is modifying an assembly. More info on my troubles with this: Modifying an assembly behaves in a similar manor to a rapid change in assembly velocity

[BusyCityGuy]

As of the above update from 2 days ago, this bug is fixed. It no longer happens in the repro file or in my game. I also tried out your repro file from your related bug report on modifying assemblies and the bug did not occur there for me either.