Syncthing does not support case-only renames when a case-insensitive filesystem (or operating system) is involved (on ANY machine syncing the folder). If you do this, you will likely encounter all kinds of ugly behavior.
This is a known limitation of the current implementation. Depending on the exact setup, there may be workarounds available but it does not work out of the box.
(If all machines share the same behavior, e.g everything is case-sensitive or case-insensitive it should work fine. Issues happen when the cross-platform behavior is different.)