If CF has to look for your custom tag (i.e., it's outside the current dir), CF will 'remember' where it found the tag. If you then move the custom tag, to either another custom tag folder or even to the current directory, CF will not be able to find the tag anymore. The only way to get CF to find the tag again is to restart the service. This is an old bug too (well, maybe not a bug since CF caches the location for performance reasons) that affects MX as well.