I just replaced a 1.6 GPF toilet with a American Standard 1.28 GPF
toilet. It received a 5 Star (best) flush rating by using a 2-1/8”
trapway and a 3” flush valve.

The first thing I noticed is that in order to get the full flush you
have to hold the handle down for a second, otherwise the flapper
closes too soon. I haven't had any problems, in fact the short flush
seems to work for liquids just fine, but it's a tad inconvenient. My
other toilet (1.6) needs just the tiniest push on the handle to start
a complete flush.

I called American Standard and they said that that's how they work.
Yes, you have to hold the handle down to get the full flush.

Anyway, I was just curious about other 1.28 GPF toilets. Same issue?

The other thing that I was wondering about was whether the small
amount of water being used is enough to move things along once they
enter the pipe. Doesn't a 20% decrease in water mean a 20% decrease
in "flow force"?

Sure, the toilet flushes fine due to the large trapway and valve, but
what happens after the stuff leaves the fixture? Is 1.28 gallons
enough to keep things moving through the pipe?