Eljer toilet circa 1996 question

noone

This house has Eljer one-piece toilets from 1996. I just replaced the fill valve and flapper on one of them and I have an issue now where the flapper doesn't shut down to the hole opening sporadically. It's as if the Korky flapper isn't heavy enough to flop back down while the water is gushing out of the holes in the circular area where the flapper and fill pipe is contained. This only happens occasionally, when you don't flush it 'just right', or if you flush it twice, only when all the water evacuates out of that cylinder. Basically on the inside of the toilet there is a round plastic cylinder in the middle of it and inside the cylinder is the flapper/drain hole and the fill valve tube where the water from the fill valve sprays into. At the base of the cylinder on the inside there are holes that the water flows out of from the fill tube.

Is a special 'Eljer' flapper needed here? Perhaps a heavier 'Eljer' flapper?

Anyone have experience with Eljer one-piece thrones?