Google Play Authenticate to complete purchase, multiple accounts
After some searching through the Github issues I found this issue regarding in-app purchases using Android/Google Play. Neto Martin, developer at Google, states the following regarding multi accounts in combination with the in-app purchases:


  The purchases are always related to the user that was selected on Play Store app when you installed the app.
  So, if an user A installs the app, all purchases will be done on it's account, even if you have the account selector in your app.


Knowing this I knew the only solution for my problem was to reinstall the app (which was my last resort as I would lose some data), making sure my primary Google account was selected, which fixed the issue for me, the app now prompts me to authenticate using my primary Google account.