This sentence makes no sense, both in italian and english. I think that the italian would be "la regola", and in this particular case "invece" should be translated as "but", and "regola" as "routine".
As an example: "I politici corrotti dovrebbero essere eccezioni, invece diventano la regola" "corrupt politicians should be exceptions, but they become the routine"
E adesso lo segnalo a Duolingo.
I am sorry that it takes a fair amount of creativity to come up with a sentence that could contain the given phrase. Thank you! I somehow doubt I will be putting this one into the long term memory banks as a "useful phrase I can employ often"!!
This sentence obviously needs some context! It would make sense if a preceding sentence stated something like "We thought that the guidelines were just suggestions."
Maybe it's about dictators? They are supposed to follow the rules. Instead, they dictate the rules, they "become the rules".
It is unbelievable that you count as a mistake the present progressive form "they are becoming." That's what 99.9% of native speakers would spontaneously say. Who in the world is supplying you with the English translations?!!
But wouldn't "they are becoming" be better expressed in Italian as "stanno diventando"? As has already been stated by me and others here, the most appropriate translation would depend on the context of this statement, which is missing.
It seems to me that the normal English expression "becomes the rule" should be used here. Apparently Italian uses the plural where English uses the singular.
Since the dictionary gives both 'instead' and 'on the contrary' as the translation of 'invece', it is perverse to reject the latter.