This is due to a mechanic specific to 1.19 called knockback scaling.
In 1.19, the knockback your opponent takes varies wildly depending on their vertical position in the air, which makes it harder to combo other players. This is exacerbated even further when your opponent has higher ping, to the point where they’re impossible to combo. Despite kitpvp being coded to behave like 1.8, it’s still on a 1.19 base, which is why this happens.
Intel Edits, a prominent 1.8 and 1.19 pvp youtuber, made a video on the subject a while ago, and it goes into more technical detail about why this happens. The main explanation starts at 3:44, but it’s really worth watching the entire video.
If 1.19 kitpvp is to be as similar as possible to 1.8, this is definitely something that needs to be addressed.