I bought with a single payment in 2017 and I can still use it

@antonpa, I think the same thing happened to me as you. The app asked me for a monthly payment despite having bought it with a single payment years ago.
To activate it, you have to select the option "I'm already subscribed", and then "I forgot the password" with the email we used to buy the app. Then you log in with email password and that's it 😃.
I'm glad that the developer takes care of the early adopters that we pay for the app. Other apps like Paste have forced single payment users to move to subscription, which I doubt is legal.