Since reference subsetting is a form of subsetting it is subject to the multiplicity rules of subsetting so inherits a constrained multiplicity.
However, reference subsetting is also used to identify ends of
connections, which ends should always have multiplicity [1..1].
There are therefore two competing rules, and the Cameo Evaluation Plugin seems to apply the subsetting rule first, which can lead to unexpected results (which has impact especially if you are using advanced cross subsetting to constraint connection multiplicities).
When using reference subsetting of a feature with a multiplicity such as [3] be sure to FORCE the end multiplicity to [1], otherwise Cameo will generate [3] instances for that end!
The Webel SysMLv2 Course now has a dedicated module on Cross Subsetting!
In addition to the many examples and exercises on basic connections, the Webel SysMLv2 course now has a dedicated module just on cross subsetting and connection multiplicities, with detailed worked examples, clear explanations, and evaluations in Cameo.