This is the Javadoc of DataStream.rescale() that describes the
behaviour:

Sets the partitioning of the {@link DataStream} so that the output elements
are distributed evenly to a subset of instances of the next operation in a round-robin
fashion.

The subset of downstream operations to which the upstream operation sends
elements depends on the degree of parallelism of both the upstream and downstream operation.
For example, if the upstream operation has parallelism 2 and the downstream operation
has parallelism 4, then one upstream operation would distribute elements to two
downstream operations while the other upstream operation would distribute to the other
two downstream operations. If, on the other hand, the downstream operation has parallelism
2 while the upstream operation has parallelism 4 then two upstream operations will
distribute to one downstream operation while the other two upstream operations will
distribute to the other downstream operations.

In cases where the different parallelisms are not multiples of each other one or several
downstream operations will have a differing number of inputs from upstream operations.