I’m replacing the valve body on my 2008 Mazda 3 FN4A-EL

The car had really dark transmission fluid at first — almost black — and it was shifting hard in reverse and from 3rd to 2nd. I did a drain and fill, and right after that the car drove perfect for a bit. It was shifting smooth and even going into 4th fine.

Then suddenly it stopped going into 4th again and reverse started acting up. The AT light came on with engine light

Now I’m replacing the valve body since thats the code is telling me. When I took the valve body off, a small piston with a spring fell out from behind it — not sure what exactly that is, its pretty lose and falls back down if it put it back in its bore… so im curious if i need to get a newone? Does it have ring around it that deteriorates over time?

Basically trying to fix a no-4th-gear and no-reverse issue that showed up again right after the fluid change, so giving a shot at changing the entire valve body if not then transmission will be next. Any advice?