What would cause HTTPS JSON data from PL/SQL to be partially text and partially binary?

We have an API call in HTTPS that returns JSON data. It works well in both Firefox and Edge. From PL/SQL, we get a response that consistently starts with text, then at predictable locations becomes binary data, then switches back to text. The binary starting point is at consistent locations (not random location), but not the same location each call. The binary boundaries appear to be a an even number, but no other power of 2 or other consistent divisor. Sometimes the entire set is received as readable (usable) JSON text, often on the first call of the day. (Perhaps