To add some data point, I was recently travelling from SYD to CDG via SIN (in Y as FB Plat), 1st leg operated by QF on AF-codeshare and 2nd leg operated by AF on the same day.

I was denied entry to the Skyteam lounge at SYD based on the fact that QF was not part of Skyteam, which I was not expecting as I was on an AF-coded flight and in same-day connection with an AF-operated flight… but as the lounge dragon said, "different part of the world, different rules" (sic, what is the point of a global alliance then?).

I complained to AF and received 7,000 miles as compensation. After further search, I still do not know whether I was really eligible to the Skyteam lounge, and even have the feeling that I would have been granted entry at the QF lounge…