Added 15% heavy cream and it separated out while incubating. Any way to prevent this?

My recipe was as follows:

- 850g whole, homogenized, pasteurized milk - 150g pasteurized heavy cream, no carrageenan or other stabilizers - 1tsp starter from previous batch - Milk and cream mixed and slowly heated to 180F for 30 mins. - Chilled to 110F, then starter stirred in. Cream and milk had not separated at this point. - Incubated at 105F for 8 hours. Checked at 6 hours and the cream/butterfat had pooled on top while the actual yogurt had started to gel underneath. - Yogurt allowed to rest on the counter for 2 hours before refrigerating overnight.

After refrigerating, the texture of the actual yogurt is pretty good and thick, but the cream had solidified on top.

Is there any way to prevent this? I hesitate to stir it in after resting on the counter out of fear that will break the release more whey before it's fully gelled.