Grenades aren't always reliable in buildings, and explosions that should ordinarily kill the occupants of a room or structure don't.

This is perhaps mainly because of the lack of shrapnel modelling, but in buildings things like pressure also come into play.

While this ticket concerns indoor use of grenades, sometimes even outdoor grenades are ineffective beyond maybe 5 feet. I've seen AI survive grenades landing fairly close. This also true with launched grenades (e.g. in the Infantry showcase).