Weird flawless groove "glitch"

[BriGuy]

I'm not quite sure if this is a glitch or not and perhaps it was intentional, but you cannot unlock the flawless groove achievement while using the solo buttons. I was getting frustrated trying to unlock that achievement which requires a 100% expert song using only upstrums.
I was using my mod which switches the solo button usage to the main buttons (as described here) (http://www.rockband.com/forums/showthread.php?t=25851) and beat it twice without it unlocking. I figured I would try it with this mod turned off and it finally worked.
It's not earth shattering or anything and I'm probably the only person this has happend to so far, but it's worth noting for anyone else trying to unlock this achievement.

[mct685]

Maybe you downstrummed during a big rock ending. That happens...

If this is true I dont think it makes sense.

[BriGuy]

I played Should I stay or should I go..which doesn't have a big rock ending and no solo sections. I played it through 3 times perfectly. I was starting to think it had something to do with hammer-on usage, so the third time I strummed everything, but still no achievement. So I switched to the main buttons and I finally got it.
It's weird, but its true. I guess they were going for realism, so they force upstrums only and no lower neck play (which I guess doesn't apply to bassists)

[Frederf]

It looks like solo buttons aren't considered upstrums no matter how you play them.