This is the case for us, too. The reason is that whenever there is more supply than consumption on the grid the excess must be shed, and that costs the utility. Hence negative export prices to discourage supply to the grid.

Our battery/inverter system automatically curtails solar generation when our battery is full and export prices are negative, so this isn’t anything we need to think about. (Exception: I’ll charge the car to drain the battery overnight if I expect curtailment the next day. No point letting all that free sunshine go to waste!)