There is a new issue related to this.
Parts that are welded or attached to characters with CastShadows disabled now cast flickery and really long shadows whenever the character is wearing a layered clothing item that also has cast shadow enabled.
This is an interesting problem because this seems to show that the engine is trying to calculate shadows on parts that explicitly have shadows disabled.
(The wings in this clip are the only layered clothing object on my avatar)
Thank you for reporting.
Yes unfortunately another completely unrelated change was made that has negative impact on shadows. This caused crashes in particular on A12 GPUs, but also has shadowing artifacts you are describing. We are working to resolve all these issues (fixes have gone in and look like they will be part of the 573 release). But it will push back the original issue a bit.
FWIW engineers are looking into this issue actively. We really appreciate getting the repros and videos.
Can confirm the original issue is fixed. Thanks again for the report!