Oil in coolant

Just so I am clear, you had this work done and now the coolant and oil are mixing? If so, I would suspect the head was not torqued properly or something else was done wrong to cause this. Here is a guide that covers this but again with this history, I would jump to the head gasket again:

https://www.2carpros.com/articles/coolantantifreeze-in-the-engine-oil

Let's test for hydrocarbon in the coolant and go from there. Here is a guide on how to do that:

https://www.2carpros.com/articles/head-gasket-blown-test

Let me know what you find and we can go from there. Thanks