i’m having exactly the same problems with all 6.+ versions and some of the 5.8.+ releases.
the last version where i can be sure to correctly edit to tape is 5.9.2.
i would recommend using this one until a correct working version 6 is released.
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4 x Avid Media COmposer Adrenaline 2.7
1 x Avid Media Composer 2.7
1 x Avid Symphony Nitris 1.6.3
1 x Avid DS Nitris 7.6
1 x Decklink HD Pro PCI-X 4:4:4
1 x Decklink HD Extreme
1 x Decklink SD Pro
2 x Decklink SD Extreme
1 x Workgroup Videohub
4 x HDLink