I usually work a lot with inclusions ( using double curly syntax, mostly closed view)
When renaming the inclusion, from the "gear" menu on the inclusion (but with the main card in the view):
see screenshot
bummed about the bug, thrilled about the bug report (very clear!)
will look into it. first guess is that this is specific to relative inclusions. (In other words, if the original inclusion were "Sandbox+new inclusion" rather than "+new inclusion", this would likely work").
potential workaround: if you rename the simple card "new inclusion" to "new inclusion Rename 1", it will probably correctly update both the inclusion and the included card.
more soon...