ILM rollover works as expected, but write-index does not move

I'm using ES 6.8.1 and have ILM configured with a Hot, Warm, and Delete phase.

When my index hits its rollover threshold (50gb), ES does trigger a rollover, and creates a new index with the number incremented.

However, I would expect that at this point, my write-index alias would get assigned to the newly created index and removed from the one that just rolled over. This doesn't happen.

Instead, the write-index alias remains on the old index, and worse yet, that index is set to read only while it reduces shard count and merges, so all indexing fails!

Is my assumption that the write-index should immediately move on rollover correct? If so, any idea why that's not happening?

Can you share your policy and index template?