I don't know if it's a bug or intended, but Withers can destroy barrier blocks, for example when they are enclosed, allowing them to escape. The barrier block is introduced as an indestructible block, like the bedrock, and it would be annoying to see the Wither destroy barriers on adventure maps.
Linked issues
duplicates 1
Comments 2
I didn't see other reports about this, yet I did a search. Sorry !
Duplicate of MC-47052 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.