In attached backtest that trades SPX bull put spread there is a huge dip in account equity on Jan 24. From the logs it seems to be due to early assignment:

Simulated option assignment before expiration - Automatic Assignment.

But what does early assignment even mean in this case, given SPX has no shares to assign?

If early assignment is disabled in the backtest, the huge dip doesn't occur, and everything works as expected.

Should QC avoid trying to simulate early assignment for index options?