We will shortly be collecting the headsets. Please place them in the PLASTIC bag.The standard pronunciation is plastic BAG rather than PLASTIC bag.
The basic rule is this: a compound noun has the main stress on the first item, so we find: POLICE car, TRAFFIC lights, MATHS teacher, and HISTORY class. In contrast, a noun phrase consisting of an adjective and a noun is stressed on the second item, for example: red CAR, tall MAN, boring CLASS, and hot CURRY. So that should be easy, right?
Well, maybe not. Why do we say CHOCOLATE cake but chocolate BISCUIT? Why do we say EASTER egg but Easter DAY? Why do we say OXFORD Street but Oxford ROAD? None of it seems to make any sense.
And then there is plastic BAG. Is plastic really an adjective? Maybe it's actually a noun. After all, we can say 'It's made of plastic', and if it occurs after the preposition of, it must be a noun.
In conclusion, it is not too surprising if there is substantial variation in the pronunciation of phrases such as plastic bag, especially in new varieties of English such as that of Brunei.