Question: fieldplot3d - Problems calling procedures that contain if statements

I have noticed that fieldplot3d can fail when calling a procedure that contains if statements.  It appears that fieldplot3d is attempting to evaluate the statements within the procedure, instead of simply calling the procedure with numerical values.

We are still on lockdown in the UK, and this has kept me amused for a while now.  But I have run out of ideas and could really use some help!

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