Not only against Highlander, it a old problem with Berserkers hyperarmor.
I don't know why this happens, but i think there is a short window between the attacks where the hyperarmor isn't active.
But sometimes it also interrupts me when i already got the hyperarmor, so i don't know if it is a bug or not ....
I stoped a Berserker combo with my Nobushi. I had revenge and since her hits didnt stagger me i hit her with a side dash heavy and stopped her combo. I think getting hit with a heavy stops berzerkers but if it was a light they just keep swinging. I'm pretty sure that HL hit you with a topped heavy and stopped you, but who knows all I know is giving an Assassin hyper armor is like give sugar to a baby they are just all over the place fidgeting.
I'll ask to see if I can get an update because I do believe we've brought it up to the team before.
And if you have a video clip of the bug you're describing please also send it along to us over at support! It'll help us if we're having problems reproducing it (which will slow the fixing process) or we'll be able to determine if it's caused by something else entirely.