Apparently the "internet" doesn't seem to have an answer to this rather simple question: Do Halion and Sindragosa have abilities that cause pushback? For instance Halion's Dusk Shround is flagged as not causing pushback. The information on Sindragosa's Frost Aura is inconclusive though.
https://www.wowhead.com/wotlk/spell=75483/dusk-shroud
https://www.wowhead.com/wotlk/spell=70084/frost-aura
Some years ago someone wrote an addon to count pushbacks, but it got lost in time. Skada doesn't count it either.
https://rebed.redditmedia.com/embed?...s%2Fes2un6i%2F
And yes, there are talents to prevent pushback, but the idea is to create a specific spec for these two encounters that tries to save exactly these points.