Just had a reply from bubble support :
Hello there! My name is Charish from the Bubble support team.
Thank you so much for reaching out! The “Text too long for this field” message is related to a known limitation when saving OAuth keys. This message is returned when trying to save data that exceeds the 2,000-character limit. Bubble saves the token returned by your OAuth provider when a user signs up or logs in with an OAuth 2.0 User-Agent flow API. Currently, there isn’t a way to increase the character limit. A workaround shared by a user can be found on the Bubble Forum thread you shared: OAuth User-Agent Flow Text Too Long for This Field.