I have also discovered that ribs can occasionally resist achieving sufficient vacuum, probably something to do with their irregular surface or my rushing and not massaging the package while sealing it. Don't know for sure but it does happen, but rarely.
If you do experience a bubble or moderate ballooning tumble the bags during the cook to move the air pocket to a variety of locations to achieve even cooking. With significant air in the package during a long cook i'd repack and continue. Floating meat won't cook evenly.
As always, during a repack practice impeccable sanitation. Consider keeping your cooking tongs and forks standing in a pot of simmering water to prevent inoculation of cooking or cooked meats.