Can you explain in more detail what you mean by "When any file (B,C,..) in the folder is updated"? What kind of update? Does this update alter the name or location of the file in any way?
Any link to an external file uses a file path to that external file--which includes at least the actual name of the file. You can see these external file references in Manage | External Data Sources.
Under no circumstances should the relationship suddenly refer to "the next link in the list of relationships". An external data-source reference can, however, list more than one file path to the same file. In those cases, if the first path fails, FileMaker will then attempt to use the next file listed in the data source reference and this is what it is designed to do here.
In the example shown two files are linked (UItgaven2011.fm and Apr.fm). When an empty version (clone) of Apr.fm is replaced by Apr.fm containing new data the link between the two files is corrupted (observed using Edit Relationship)
What do the entries for these files look like in Manage | External datasources?