How can I prevent UNPIVOT from being transformed into UNION ALL?
Try the following Oracle optimizer hint:

NO_EXPAND


This hint disables OR-expansion. OR-expansion transforms combined OR-conditions or IN-lists in the where clause into a compound query with the union all.

We don't have your SQL so this is a guess but seems to a viable option.  

As a second option, since you are in a development environment, you can try the Oracle optimizer hint:

NO_QUERY_TRANSFORMATION


but this hint disables all query transformations that the optimizer can perform other than transformations that the optimizer always can apply.