It matters. For OAuth (Google, FB, etc) there is no distinction between sign up and log in (this is not a Bubble thing, but an OAuth thing).
As a result, when using OAuth, before authentication, you will not know whether the user had previously signed up or not.
This results in the following scenarios, for different combinations of email sign up and/or email/OAuth login.
If the user has previously signed up with email, they have to be first logged in with email to subsequently also use OAuth for the first time they use OAuth, if the OAuth account is associated with the same email id. Once this is done, thereafter they can log in via email or OAuth.
If the user has previously signed up with email, and they are logged out, and if they try to use OAuth, Bubble will show the error (email already in use). The only way forward is to log in with email, then do OAuth. But as said above, this only needs to be done in this order once.
If the user has never signed up with email, and signs up with OAuth for they first time, they cannot later use Bubble’s “sign up” workflow for their email.
All the variations you see can be traced to one of these three scenarios. Any deviation is because the development and live DB is different. The live DB sign-ups are independent of development DB sign-ups.
In my Bubble app, if a user initially signs up with their email, and they go to the settings page, they see a button that lets them add OAuth, and subsequently they can use either to log in.
It is a good idea to send a confirmation email to the user which tells them how they signed up (email, FB, Google, etc) on their first sign up. You can also create fields or use Bubble’s auto-generated fields to keep track of the way in which a user has authenticated. For example, user’s Google is not empty, etc.