Commentary for Avodah Zarah 151:6

While it is true that permitted substances were cooked in the Temple vessels, when they discharge taste, that taste is from a substance that is now prohibited. The taste has been in the walls of the vessels for several days and sacrifices become strictly prohibited after one or two days. Thus in both cases, the substance cooked in one of these vessels would have the taste of a prohibited substance.

Rava explains that the word “boiling” does not mean what it seems. Rather, it means rinsing and washing, two verbs used in the context of cleaning holy utensils in Leviticus 6:21.