The science of lawn mowing has been thoroughly accessed by much more learned heads than you or me!
That said---The engineers who designed the machine have designed a good machine, in that they have spent months/ nay years, designing their machines to mow your grass with perfection! You have sort of upset the apple-cart by installing blades that obviously do not do the job of mowing very well. Unless they are upside down, which would put the dull edge against the blades of grass. Put the original blades back on and mow!
Installing the blades up-side down will surely affect the mowing, as the blunt edge of the blade will not mow well.

The beveled edge of the cutting edge faces upward. The completely flat side of the blade faces down.

"IF" the blades were on upsidedown, then it wouldn't cut
in reverse either, as the blades don't reverse direction, do they? Something very strange here.
I think it best not to upset the Lawnmowing Gods.

At this point, I'd be questioning whether you were supplied with the correct mulching blades for your tractor model. Since they don't work, why are you not going back on whomever it was that sold them to you with this complaint? I sure as heck would.

IMO the cut maybe terrible with the mulching blade due to mowing too fast. I have found with mulching blades they tend to blow the grass down more and you have to slow down to get a good cut and let the blades pull the grass back up especially if you have several different types of green stuff in your yard. Try slowing down. Need to slow down anyway to get the full effect of multching anyway. when you backing up the grass is being pulled the other way and you have already mowed over it so it should be better being you've cut it twice and you are going slower in reverse IMO.

I'm confused? The "Gator" mulching blades I put on my GT 500 (Sears) Have a decided curl on the back of the blade to push the cuttings up against the bottom side of the deck. THey then fall back and are "recut" hence the term mulching as they are chopped rather than just cut and discharged. I'd suspect you have the wrong blades, perhaps "bagging" blades as opposed to mulching.

When the blades are mounted correctly, the "sharp" end of the blade is the lowest portion of the blade. It's easy to put them on upside down if you have the deck off and it's upside down.

Certainly, speed makes a big difference, the slower you mow the better cut you achieve. I've noticed it myself, mowing in second or third gear as opposed to fourth or fifth there is a decided difference-even with newly sharpened/balanced blades. That being said, I mow almost 4 acres and don't have the time to creep around trying to get a golf course.

Would be interested to learn the final resolution.

Ev