From description in Appstore:

*Please note: Halide is not free. We do offer a 7-day free trial for annual memberships. Don't like subscriptions? You can also buy the app outright with a single in-app purchase.*

Is that what you did?

I bought Halide v1, which was an upfront purchase.

They grandfathered all existing v1 purchasers, but the grandfathering somehow stopped working for me because I didn't buy it on my primary App Store account (I switch between regions, but pay for my non-free apps using my US account because they're usually cheaper).