Did your APR increase after 30 days?

It may change any month. See the following from the Apple Card Customer Agreement https://www.goldmansachs.com/terms-and-conditions/Apple-Card-Customer-Agreement.pdf

Variable APRs may increase or decrease each month if the Prime Rate changes. Your Purchase Transactions APR is a variable rate. If the Prime Rate changes, the new variable APRs will take effect as of the first day of the next month and apply to existing and new balances. Any increase in the Prime Rate may result in an increase to your interest and Minimum Payment Due. APRs will never exceed the maximum rates permitted by applicable law.

The "Prime Rate" in effect for a given month is the highest U.S. Prime Rate published in the print edition of The Wall Street Journal (WSJ) on the last day of the prior month that the Prime Rate was published. If the WSJ does not publish the Prime Rate on that day, then we will look to the last day before then that such rate was published. If the WSJ temporarily or permanently discontinues publishing the Prime Rate or if the WSJ changes its definition of the Prime Rate, then we may substitute another publicly available index not within our control and adjust the Margin, in our sole discretion, subject to applicable law. However, for the month of August 2019, the Prime Rate is equal to the highest U.S. Prime Rate published in the "Money Rates" section of the print edition of The Wall Street Journal (WSJ) on August 2, 2019.