Take my 280,000 points award. The points cost for all individual flights, added together, would be well over 400,000 points. Even if I had one or two Y segments.… The points component of an award booking will always be the lowest possible price - either the through fare if using airlines all on the one table, or the sector-by-sector fare is using airlines across different tables OR where mixed class is involved.
In many cases, where mixed classes are involved, it will usually be cheaper for the through fare to be applied ...
...but that's not always the case. SYD-DFW in F = 189800 points. If QF re-routes you via LAX, so you're SYD-LAX in F connecting to LAX-DFW in economy, the sector fares would be 162800+18000 = 180800, so you'd be entitled to a refund of 9000 points.
I won't be getting (or expecting) any 'discount/refund' due to Y flights.