A trunk route that does not connect to another trunk road or motorway is a trunk stub/spur. The trunk section of Soldiers Field Road that you reinstated and extended only terminates at a primary route when trunk roads are intended to link to other trunk roads and motorways to form a fully-connective network. 

In an urban scenario like this, trunk roads would be the major commuter routes for getting across town and should not abruptly end at roads of a lower classification. SFR continuing east from the Eliot Bridge is much more important as a through route, connecting Fresh Pond Parkway to the MA Turnpike before becoming Storrow Drive and heading towards Downtown, while the section east of that junction simply terminates at Beacon Street without a connection to any other major highways.