I'd tend to think that's a bit beyond the scope of GridStatusMissingBuffs, especially given that detecting who is a tank isn't cut-and-dry, even in a 5-man. The LFD role is not a forcible requirement. My druid, for example, is a tank in raids, but I queue as DPS to get in some kitty playtime. On several occasions, I've ended up tanking anyway, because the assigned tank was bad, or the healer was bad and couldn't keep the tank up and my gear was better.
It's also annoying when addons have features that work one way sometimes (in your example, in a 5-man) but another way, or not at all, at other times (in a raid).
Open GridStatusMissingBuffs.lua and add print(status.text) on its own line just before the line causing the error. It will probably generate large amounts of spam, but it will print the name of the buff group currently being processed to the chat frame... when the error occurs, see what the last thing it printed was, and you should have you answer as to what the problem is.