Callactivity with Expression always use value of first evaluation

If there is:
Process1 calls Process2 multiple times (e.g. using multiInstance).
Process2 has a CallActivity with an expression as calledElement (e.g. expression: ("subprocess-").concat(a) where a is incremented each call of Process2).

During Runtime the expression is only evaluated once (at first usage). Afterwards this value is re-used, instead of evaluate the expression again.

Therefore Process 2 always calls subprocess-1; but not: first subprocess-1, then subprocess-2... as I would expect.