This headline may be "wrong" in your eyes but this is the way that inflation is universally reported. Even finance centric news sites such as Bloomberg or WSJ report inflation in this way. It is universally understood to mean year ending.
You're just not right. Saying prices rose 8.5% in March means prices rose 8.5% in March, not in the year ending in march. Reputable journalists say inflation rose to 8.5% in march, which isn't the same as saying prices rose 8.5% at all. I really don't think this is petty at all. Saying prices rose 8.5% in a month is incredibly misleading.
My comment isn't that the wording is right. It's that there is no ambiguity on what is actually being reported. This "issue" only exists in the mind of grammar warriors on message boards.
That's how you know you are talking about a first-world country. My dear home country had an official (likely underreported) inflation rate of 5.5% in March alone.