Yes, the ID is different when you invert them, when you created your object it should have returned it’s associations and their different ids. In the json, there should be a “From” and “To” key for each pairing. You have to make sure the ID you are using from the proper direction.
Example:
{
"fromObjectTypeId": "2-3444025",
"toObjectTypeId": "2-555555",
"name": "cat_to_dog"
}
If you dont have the json, you can run a “Read” request on your object to get the json again. Or, In my experience, the id will normally be 1 off of your current use case (so likely 16 or 18), so you could try to just update it that way and see if that works, but this isn’t always the case.