Since I switched from Calyx to iodeOS all my in-app purchases via Google Play automatically get refunded after a week or so. This is not that big of a deal for me right now but this should be addressed. It’s also kinda funny because this way I could just purchase coins in a game then spend them and after it got refunded they just deducted the amount again and now I have a negative value 
You are logged into Aurora with your Google account?
I help 1 family member with a similar situation (they have a purchased game from Google Play Store) that hasn’t reported any issues with the game, but I think that they do not do any in-app purchases. So in your case do you have paid apps too, and do they work, or do you only have in-app purchases?
Another question is if you restored Aurora settings from SeedVault? Possibly you could try a cache deletion of Aurora, then re-login and confirm if you still have the issue.
Yes I am logged in with Google in Aurora (via their microG integration). I’m unsure I might have restored it with either SeedVault or Swift Backup.
Paid apps work normally. License checking succeeds.
I can try to clear the data and purchase something that I intended to. Do I need to reinstall the app where I want to make the purchase too?
Gotta say I’m a bit confused about the technical level of how Aurora might affect in-app purchases. I see the issue more on the microG distribution of iode.
I think before doing any resetting then it may be worth reviewing at microG. Coinciding with you switching ROMs was the shutdown of SafetyNet servers, etc., so it could be recent Google changes that have caused the issues? There is this one at first glance, but you may want to dig through their issues a bit more: About game purchase and recharge issues · Issue #2642 · microg/GmsCore · GitHub
One more confirmation: Your microG “Play Store services” are all enabled? (System Settings > System > microG > Play Store services)
all enabled yes