If I have a stored procedure with variable result sets (and therefore no RESULT clause), what are the requirements to use that in the FROM clause?

Background: I have a stored procedure SP1 that returns different result sets based on one particular input parameter, kind of a "test" flag that when set selects several result sets and otherwise just one (all with different schemas).

AFAIK the system catalog (i.e. system view SYS.SYSPROCPARMS) does store the result set returned by the first SELECT statement in the procedure's body. In my case, that one belongs to the "test" result sets.

Now, another procedure SP2 calls that procedure within a SELECT FROM ... and does not set that "test" flag.

According to the missing RESULT clause and the fact that the result set stored in the system catalog is different from the one I'm expecting here, I certainly have to provide the expected schema via a fitting WITH clause as part of the procedure-call, i.e. something like

   SELECT *
   FROM SP1(...) WITH ([expected result set description]) AS SP

While this has worked several times, now I do get a SQLCODE -866 ("%1 returned a result set with a different schema than expected") error.

I cannot resolve this via recompiling or altering the according procedures.

Have I somehow missed the point, or are there further requirements to take care of?

I'm still using v16.0.0.2798.