Please provide an example file demonstrating the problem.

Here is an example blend file.

I don't think that this is a "bug", more of a result of how the slow parenting code is implemented - parent relations in the Blender editor which have a time offset value are assigned slow parent relationships, which are handled differently to other parent relations, and we don't offer the ability to create a slow parent relation in the BGE at runtime. Possible solutions:

Give slow parent behaviour to all parent relationships, and set default time to 0.0
Allow timeOffset setter to create a slow parent relationship.

This patch doesn't fix totally the issue but maybe you can use it as starting point to fix it correctly. I have uploaded it because after a lot hours I don't find where is the fail.

Then the issue with the patch is:

• If you set parent relation only at children level you got the slow parenting working but you can't set/get timeOffSet new values because they are done at parent level and it won't work.
• If you set the parent relation only at parent level the slow parenting won't work but you can change timeOffSet values but they won't have any effect.
• If you set the parent relation at both children & parent levels it works like a charm but you will have a crash on exit when the program tries to delete m_parent_relation on SG_Node::Destructor().

Also, as remark Moguri saw that in the Blender->BGE conversion data function a parent_invert node is created between childre and parent.

Regards