UMLR — Creation of an expansion node under an activity is allowed by UML and SysML specifications

Which semantic for an expansion node owned by an activity (instead an expansion region)?

According OMG Unified Modeling LanguageTM (OMG UML), Superstructure, Chapter 12.3.26

An expansion node is an object node used to indicate a flow across the boundary of an expansion region.

An expansion region is a structured activity region that executes multiple times corresponding to elements of an input collection. This specific structured activity node is using expansion node as input and output. From outside the expansions regions the elements of expansion nodes only appear as a collections, the elements of collection are only accessible from "inside the collection".

Semantic of an expansion node owned by an expansion region is then well defined. However, in abstract syntax nothing prevents to create an expansion node owned by an activity instead of an expansion region. In this case semantic is questionable.
If this kind of construction is not expected, a specific constraint should be added in UML specification in order to prevent an activity to owned expansion nodes. On the contrary, if this construction allowed, associates semantic should be defined.