Thanks for posting all the details.
I’ve found a bug in our internal tracker that sounds like the issue you’re describing. Fortunately, it has already been fixed and the fix will be included in the next version of CS-Cart, 4.19.1 The bad news is that this version will not be released until next year.
In that case, I can offer you the .diff file with the necessary changes, please apply it to your installation to fix this bug.
ebf28a348e25ff0cf3e65ccbd07a5e5e8b74dc21.diff (9.0 KB)