Blocks do not break with swords while in creative since 1.6, it's intencioned, so that while fighting enemies in creative you do not accidentally break blocks.
With the conditions I described, I can't reproduce the bug, so I suppose it was caused by an accidental typing of mute like in MC-29454. I will report if the error/bug reproduces again. Thanks for your rapid answer.
Yeah, it is a duplicate, sorry