I was playing around with the new AssetService:LoadAssetAsync() function announced a few days ago, and when loading models (at least older ones) I noticed that the welds were not preserved correctly, causing the models in question to fall apart. When I loaded models that fell apart in-game into studio via the toolbox, everything worked as intended, leading me to believe that this is a bug.
Example
Here’s a random model that I have, loaded into the game from studio via the toolbox:
And this is what the same model looks like when the game attempted to load it with :LoadAssetAsync(34705016)
NOTE: I did try to manually replace the welds with WeldConstraints, but it seems like it doesn’t even preserve their part0 and part1 data, if it preserves them at all. Even then, as mentioned previously, the toolbox can load them just fine.