This would be a different scenario from the OP's situation; althought my friend had a very different experience when she was in your situation. This was maybe 1 year before COVID. She had a UK passport, an Aussie PR. SQ checked her all the way from Europe (I think maybe Spain) to Sydney. She landed in Sydney, was taken by Border Force into a room, because her PR visa (which is only valid for 5 years as you said) lapsed. She said she was given a 3 hour lecture by Border Force in the room, before being issued with a one time only entry visa and a warning letter. I remember I read that letter and I found it very funny, which she didn't find funny at all she said. Now I wonder how did SQ check her in if she didn't have a valid Australian visa (and not Aussie citizen)I understood there is a check to see if you have a visa to enter Australia before you board if you don't have an Australian passport.
(I was caught on this some years back when travelling on my UK passport with a >5y old AU permanent resident visa which needed a right to return. IST check-in knew and would only book me through to KUL. Thankfully eventually sorted out by MH though very stressful)