FWIW the terms and conditions state you have to active the complimentary insta+ in order to get the $15 credit. (at least the CSR) and as a data point the first time my wife used her card she didn't get the credit, the next time she activated the I+ and it posted perfectly every month since. Even two orders with a partial credit that were under $15 (Aldi pickup order cancelled some items so it dropped down to like $11 which credited). Made another pickup order a day later and the $4 credited.

Cut and paste from chase website Statement Credit on Instacart Purchases: Earn up to $15 per calendar month (for Chase Sapphire Reserve and J.P. Morgan Reserve cardmembers) in statement credits on qualifying Instacart purchases from 8/1/2022 through 7/31/2024 when you activate a complimentary Chase Instacart+ membership on Instacart.com/chaseopens overlay with your eligible Chase credit card.