Might be a problem with the contacts of the heart rate monitor, especially because you mentioned that the effort didn’t feel easier and you didn’t notice any problem with your HR.
I had the same problem with my Tickr a while ago, my HR always started dropping mid workout. I contacted the Wahoo support and they send me a completely new tickr which solved the issue.
Concerning the training adaptations I don’t think your going to face a issue. As far as I know AT only considers power data an not HR. So as long as you haven’t marked the workout as very hard in the post workout survey you should be fine.