Why did it raise a bid on a keyword with a high ACOS?

Counterintuitive, but legitimate. High ACOS with a very low bid often means the keyword is stuck in bad placements — winning only cheap, poorly-converting impressions. The math says it can afford more per click than it is currently bidding.

Other causes: conversion rate improved since the last optimisation, or a placement modifier changed and the base bid needs to compensate.

The safety check: for Sponsored Products, a raise on a high-ACOS keyword should still land below the historical CPC that produced the high ACOS. The proposal detail shows both numbers so you can see it.