It's because of the switch to a cached_property ; some code (mostly tests I think but I didn't want to take risks) expect the following to work:
```
assert not addon.payment_account
AddonPaymentAccount.objects.create(addon=addon, ...)
assert addon.payment_account
```

To avoid having to change the existing code I don't know much about, I have this, which resets the cached property when you save the `AddonPaymentAccount`.