Hi,
I guess this is a combination of ACL misconfiguration and behavior of MS Office (I assume you are using MS Office).
The later opens a file for editing. When you write, it creates a new file, writes content into the file, deletes the original one and renames the new one to the old name. See here.
When a file is created it inherits the owner and permissions of the folder. This is the reason why the file appears to change its owner- because it is a new file.
So you would have to check for the folder your ACLs (from Windows) as well as your share permissions (in UMC). You might want to reset your ACLs in case you initially made a mess.
/CV