I don't think it's sneaky in the same sense as LH has deliberately pushed many mainline flights onto Germanwings to avoid having to provide lounge access and mileage credit to other *A FFP members. It might be an unfortunate by-product, but it doesn't look like that's what A3 is trying to achieve here - it's more efficient utilisation of the available fleet.

The position with A3 & OA flights is complex and evolving, with many substitutions/switches, and a definite blurring of which metal is used. For an A3 cardholder it makes little practical difference - in fact, in many cases it's a benefit because flights that were OA-coded and therefore Y-only configuration are now being operated by A3 which has opened up a J cabin that can be upgraded into using vouchers. For others, in the short term, it can lead to confusion everywhere - including, as you've seen, with the staff in how they apply benefits.

The sooner OA becomes at least an affiliate member of Star, the better - because right now this is just messy. Unfortunately no-one really knows whether that's on the cards or the timescale for implementation if it is.