Charged Interest for Unbilled Purchase?

[Brax]

I have been carrying a promotional 0% APR for 15 months supercheck balance on a major bank credit card for several months. This has been the only balance on the account, the last purchases I made were much earlier in the year and were PIF completely. Logically, I haven't incurred interest while carrying the promotional balance and I always paid more than the minimum payment each month.

Two weeks ago, I made a purchase halfway through my statement cycle. I wanted to pay the purchase off, but my belief is that doing so would first go towards the billed promotional balance BEFORE applying to the unbilled purchase. So I was planning to PIF the purchase as soon as it showed up on my statement which just closed.

Confusingly, I was charged interest on the purchase from two weeks ago. This is the only purchase I've made. It's as if carrying a 0% APR promotional balance elminates the minimum 21 day grace period recieved on purchases. Is there some rational explanation for this situation or could this be something illegal?

[too-much-time]

A BT dilemma may be of interest to you, so to speak.

[missjae]

When I carry a balance during a 0% interest period, I try to avoid using that card for purchases because the way the company applies payments is confusing. It can result in being charged unwanted interest rates. Do check out the thread too-much-time suggested. It was informative for me.

[Dustink]

Anything above the minimum payment is applied highest to lowest interest.