Using the (array) helper creates an array instance, passes it to applyValueTransformer, which then allows themes/plugins to mutate it.
When the helper is re-computed, the **same array instance** is passed to the transformers again. Any elements added in the last run are still there.
This commit moves applyValueTransformer to a getter, so that it's run with a brand new array each time.
03bb43f746