I just installed three Koehler’s with the extra built in flush. Still have to swab it always with solids.
Hold the lever down for 3-4 seconds to dump the entire tank during a flush. The flush tube drops too fast.
The 1.27 GPF are about the ragged edge for flushing.
Had a customer with the Hi-Lo twin flush toilet. (Lo #1 and Hi #2)
All is good until a house guest hits the wrong button after taking a dump.
Yes, its because there is not enough water “surface” in the bowl.
Check to see if there is a little restricting orifice on the bowl fill tube that comes out of the fill valve. (Runs down the center of the flush tube.)
If it is a small orifice, drill it out with an 1/8” drill bit and reinstall it. Should put more water in the bowl between flushes.