The 75% scaling on Fukua's j.HK works the same way as the 50% scaling for throws/hitgrabs. You wouldn't see much difference if the scaling was changed to not affect mid-combo j.HK. The 75% damage scaling is a MINIMUM scaling. If your combo is already scaled to less than 75% the extra scaling on j.HK has no impact at all.
Combo damage scales like so
1st hit 100%, 2nd hit 100%, 3rd hit 100%, 4th hit 87.5%, 5th hit 76.5%, 6th hit 67.0%, 7th hit 58.6%, 8th hit 51.3%
So if you insert the j.HK later than the 6th hit in the combo it will have zero impact on the damage of your combo. Even if j.HK is the 5th hit, the extra scaling is minimal (an extra 1.5%).
You can see the same effect with Squigly's Silver Cord (50% damage scaling). If you insert Silver Cord into your combo on the 8th hit or later, it doesn't add any extra scaling to the combo.
Test it in training mode for yourself
cr.LK > cr.MK (all hits) > s.HP, j.HK, s.LP <---- the s.LP does 175 damage in both the beta and retail versions. No difference.