They mentioned in the recent Reddit AMA that they think they’ve got it pinned down:
https://www.reddit.com/r/hearthstone/comments/ay645h/hearthstone_developer_ama_year_of_the_dragon/ei0v1ht/?context=3
It’s been one of the more elusive bugs for us. We’ve finally been able to reproduce this bug more consistently and think we have a fix for it.
Evidently it didn’t make 13.4 but hopefully this one will finally be fixed soon.