To be expected actually.
Imagine the most ideal vaccine scenario.
Lets say a vaccine has caused T-cells to recognize a virus and what antibodies kills it best.
Then later the vaccinated person get that type of virus in its bloodstream.
The immune system still can't know its there.
The immune system does not even have a hint until AFTER the virus has entered a cell and started killing it, so it sends out exosomes as a warning.
But that only means the immune system detects these exosomes.
It still has to manufacture antibodies and they have to search all over, looking for the infected cells.
So any immune person can still test positive from an infection that the immune system has no yet found or wiped out.
Then consider how much harder it is with covid, since it hides in airways instead of the blood.
Then imagine the additional risks from the fact with covid, the immune system over reacts and does far more harm than the virus.