Assignment rules on Instance id should be quoted strings. But, if the value is unquoted and numeric, the initial assignment during export will fail due to being the wrong type. SUSHI sees that the value is numeric, and makes the guess that maybe it's supposed to be assigning an instance identified by that numeric value. It will fish for such an Instance, find the one already being exported. This is because the Instance's id accessor method casts the rule's value to a string. This leads to the Instance being exported again, eventually leading to call stack overflow. It would probably make more sense if a non-string value on a rule on id did not get returned by the accessor.

Sample FSH:

Instance: requireddataforantimicro-response
InstanceOf: Questionnaire
Usage: #example
* id = 3193360

In this case, the expected behavior would be an error on the id rule indicating that the numeric value is the wrong type.

Zulip thread