thank you for adding your feedback and for confirming my thoughts.
I would now create a MDLSITE issue to let David think about changing this behaviour for plugins, but before that I am wondering if plugin language strings are handled differently from core language strings during auto-propagation. Normally, I don't translate core language strings myself after they have been auto-propagated, so I simply don't know.
Thus, dear core language pack maintainers, can anyone of you tell if this also happens for core strings? If not, this would be another argument to have the behaviour changed for plugins.