Whenever you trade long (debit) or short (credit) equity options spread, the short leg can be assigned at any time. Why is this? Well, equity options, for the most part, are American-style. That's not because they trade on American exchanges, but rather, the method in which the owner of the option can choose to exercise their long option, which results in an assignment if you were short the option.

First, long options do not get automatically exercised if you were assigned early because all exercise requests occur overnight. As a result, we (tastyworks) do not know that an account is assigned shares until the morning after the counterparty's exercise request. However, the only time a long option automatically exercises is if it expires ITM.

Furthermore, automatically exercising long options after assignment can potentially cost you more money, and that's because of any residual extrinsic value that the option may have.

That said, if you are assigned early then, you can perform a covered stock order to close the assigned position by selling the assigned short or long shares with the corresponding long call or put. In other words, the theoretical max loss may reduce by performing a covered stock order due to any residual extrinsic value remaining in the long option.

To learn what happens to options spread at expiration, please click here.
To learn about performing a covered stock order after being assigned early, please click here
To learn about reconciling your profit/loss after an assignment, please click here.