That's very weird, though there was a spell when it didn't seem to be automatically adding new books to the series page - that was cured (for the UK) when the new series thing came along and you could add them yourself. I've looked at the Australia results for my 21 book series and there isn't a series page at all. But I can't say I've ever checked that outt before.
The new series thing doesn't appear to be working for the UK. The ones missing are the ones since the new facility came in.
I put in a 'fix this' note to KDP, and noticed that only US, UK, and DE stores have series pages. I asked for AUS ones as well.
It's no wonder my sales in Aus are less than 5% of my total, when the store doesn't display the full series at all. It's too hard for people who discover the latest book to go back and buy the previous ones.