100% a bug - this boss is one of the easier bosses to solo, since his attack patterns are very predictable and easy to dodge. Many bosses have attacks where they basically warp to you or turn instantly and hit you right away, which you can not reliably dodge.
Normally the wights come in waves as you get the boss down to 75%, 50% and 25% of remaining health.
Which is the only conclusion my statement implies about those who are having problems with it. What you are concluding with your faulty logic in your brain is your problem, not mine.