It is possible that they don't have the proper enzyme to break the 1-6 linkages, but then they would not be able to break them in the starch either. Both starch and dextrin are made up of glucose linked through either 1-4 or 1-6 bonds, the difference is starch is bigger. So if they can break down the starch except for the 1-6 bonds, they can do the same to the dextrin. They will break them down the same way.
Alpha and beta amylase both break 1-4 linkages, they just vary in where in the chain they break the bond. Something like limit dextrinase will break 1-6 bonds, that is found in barley. Whether brett, pedio, or lacto has that or another enzyme that does the same thing, I don't know.