I want to start by noting that this issue occurs only under specific circumstances. It specifically affects welded models that are stored under ‘Replicated Storage’.
The issue arises when a welded model from ‘Replicated Storage’ is duplicated and placed in the ‘Workspace’. Upon initial placement, moving the model from its original position in the ‘Workspace’, and then adjusting the welded part, if the ‘Undo’ action is used, the welded part reverts to its original position in ‘Replicated Storage’. Notably, this reversion does not occur if the model is moved a second time.
Attached is a video showcasing the bug for further clarification.