Google tasks are all-day tasks
In the current Google tasks api the "due" field only contains date information; the time portion is fixed at 00:00 UTC. Treating it as a meaningful UTC date-time causes KOrganizer to display the task at an arbitrary time, and often on the wrong day.
For example, I have a Google calendar set to Central time. If I use the web interface to create two tasks, one due Dec 5 at 09:00 and one due Dec 5 (all day), the API reports both to be due at 2020-12-05T00:00.000Z. If I sync them, KOrganizer displays both of them on the 4th at 18:00.
Treating Google tasks as all-day tasks reduces the problem. The all-day task is displayed on the correct day. The 09:00 task is displayed on the right day, but the time of day is still lost.