I have simplified the drawing to:
@startuml Call Policy Set 4
start
title Number Analysis
switch (source)
case ( a )
:case1;
case ( b )
:case2;
end
case ( c )
:case3;
endswitch
@enduml
If the stop is moved to either the first or last branch the the drawing is rendered correctly. The same occurs if stop is replaced as end.
The exception thrown is:
I have tried this in release 1.2021.00, 1.2021.5 and 1.2021.6b6