This is what often happens to your Amazon keyword rankings when your most popular child ASIN goes out of stock.

Usually, profits decrease and TACOS increases. 2 reasons:
1. Ad conversion rate decreases because the most popular variant isn't available

2. Organic sales decrease because the organic ranks have dropped

Good agencies and ad managers act quickly to lower the spend on that parent ASIN when such situations occur.

They proactively control the TACOS and minimize overspend.

Lesser agencies and managers keep spend on autopilot, and usually aren't even aware of the ranking loss.

Because 1) they don't understand the client's inventory situation, and 2) they don't even track organic ranks.

Being truly effective with Amazon ads is all about spotting things QUICKLY and adjusting.