I have tried with Databricks Runtimes of 14.3 and 16.0 with the same results. I wouldn't like to use anything prior to 14.3, so don't want to try earlier versions.  Remember the computes all support the coalesce, its just that using it with a DBR instead of a SQL warehouse compute, the coalesce causes different behaviour. Specifically it stops respecting data types coming through the coalesce when using a DBR.