I can reproduce the behavior. I opened a random .cpp file, typed "/*!" ENTER, typed "\". Each "\" creates new /** style comment block below the current one.
The "\" key is bound to c_backslash.
I stepped through c_backslash, and it seems to do almost nothing (based on code inspection and F11 stepping into each line). It calls call_root_key, which in turn calls keyin and gets an index of 0 so it doesn't call anything else.
I stepped out of c_backslash, and the comment block appeared.
The keyin call looked interesting at first because F11 shows that it somehow triggers a call to commentwrap_doeditkey, but stepping through that seemed to do nothing.