iOS 5.1.1 unable to play a MP4 with single track subtitle

Hello.

I find a problem with iOS 5.1.1

I have iPhone 4 iPod touch 4 and iPad2 all of them are updated to iOS 5.1.1

them I found this bug!

open video and play a MP4/M4V moive with single track subtitle.

there is no way you can display the subtitle track.

but you can display the subtitle on iTunes and Apple TV (5.0.1)

If you insert another subtitle track into the MP4 file. them it's OK on all device!

by the way I don't have this problem on 5.0.1

I hope there is a develeper who can submit this bug to Apple!