Ultegra R8020 STI shifter sticking
It’s possible the shifter mechanism has some material in it that is obstructing the proper operation, maybe a bit of wire from the cable, maybe dirt or grit. Unfortunately it may also be that something broke in the shifter.
You’ll have to systematically track down the issue. I’d start by removing the cable completely so you can check for damage to it.
Next, work the shifter without the cable installed just to check that the leavers are not jammed.
If the cable was not damaged re-thread it through the derailleur and out of the housing on the handle bar, or even the back of the shifter without going through the housing if you can manage it. Make sure the cable head is seated in the shifter properly. Put a little tension on the cable and work the shifter to check it’s operation. You may be able to get a better sense of what’s going on with it.