Hi, thanks for the report!
This interaction is behaving as expected. In the Sith raid, the bosses like Nihilus have a special ability called Fearsome Foe. For Nihilus, it reads:
"As long as this unit isn't Toppled, it has +1 Bonus Action (with a 50% chance to gain an additional Bonus Action each turn)"
Now, Darth's Sion's Unique ability "Lord of Pain" says:
..."At the start of each enemy's turn, if they have Pain, Sion gains Taunt until the end of the turn."
The key takeaway here is that Nihilus is getting 2 (or 3) separate actions, but is only taking one turn. At the beginning of that turn, Lord of Pain triggers and Sion gains taunt. Nihilus' first action is to attack Sion and remove the taunt. However, at the beginning of his second action, Sion no longer has taunt, so Nihilus can choose a new target. It also is not the "beginning of turn", so Lord of Pain would not trigger again and reapply Taunt to Sion.
Hope this helps!
