Toto Ultramax Round Front Flushing Issue

We installed a Toto Ultramax Round Front toilet almost a year ago on the advice gleened from this forum. We really like the toilet and have not had any issues (since a kid flushed a pencil the week after we installed, that is!) until the last couple of days.

At the flush, everything whooshes down in that wonderful Toto fashion, but then there is a 2 or 3 second pause before the bowl starts to refill again. There is a rush of bubble around the base of the contraption on the left inside the tank (sorry, don't know my toilet terminology) as the tank refills. We never had the "pause" between flush and refill before. Of course, nobody in the house owns up to having "done anything" to the toilet.

I should mention that we are also getting a backwash into the bowl of some of the mineral buildup in the tank (we have high iron and magnesium which our water softner doesn't catch). It looks like black scummy flakes.

Any ideas? Thanks!

There's a seal in the top of the Korky fill valve that might need replacement
https://terrylove.com/korky/