I don't get it neither.
That's how EY lost me as a gold FF
Could not book a J ticket BKK-BRU whilst fully open on Expert flyer on both segments individually. Managed to upgrade 2 segments with miles (the last segment with miles from outbound travel!) and 2 segments with plusgrade bidding system, then ran into luggage overweight problem at check in because plusgrade upgrade does not allow the extra 10 kg for J.

The ticket was certainly not worth the expense and the hassle combined.

That's why I fly QR this year, 4th return BKK-BRU this month and I upgraded return with QR credits from reaching gold, so far so good ^