The single biggest cause of shift problems like yours is cable friction, with the second being friction within the lever. You're probably familiar with cable friction and hopefully ruled it out.

The lever friction problem happens when folks lubricate or rebuild ergo levers. The mechanism rides in the spring carrier which is free to rate a few degrees within the body. They do this to create over travel when you downshift, and the RD spring then pulls the ring back to the stop.

The ring rides in a close fitting hole in the body, which should be dry. When folks over oil the lever, the oil or grease wicks between the ring and the body, and acts like sort of a hydraulic damper preventing the ring from rotating back promptly, leaving the RD in a slightly overshifted position.

The easy diagnostic is to down shift and wait a second, then if the trim is off gently push down on the thumb lever. If that corrects the trim you need to fieldstrip the lever, clean the spring ring and pocket, then reassemble with grease between the G-springs and central cam, but no grease or oil outside of the ring.