Exporting ICML with cross-references on macOS
1. InDesign CC 2019 for macOS
2. When you export any ICML on macOS containing a cross-reference,
3. The ICML should contain a full path to the referenced file.
4. Instead, the referenced file's name is included, but not its path.
Cross-references exported as ICML on macOS get a simple filename and not the full path to the destination. When the ICML is placed, the cross-reference will be broken because the file pointed to will not be found. (Unless the ICML is in the same folder as referenced document or some subfolder of that.)
John Whitney commented
Note also that cross-references inside an ICML-linked story break every time you update the link, unless the referenced document is alongside or in a parent folder of the ICML file. (And again, this behavior is macOS-only.)