Hello Aliber,
Welcome to Apple Support Communities. You mentioned your default calendar so may have checked it on iPhone already. If not, check to make sure that your Google calendar is set as default using these steps from Set up multiple calendars on iPhone.
"Set a default calendar
You can set one of your calendars as the default calendar. When you add an event using Siri or other apps, it’s added to your default calendar.
- Go to Settings > Calendar > Default Calendar.
- Select the calendar you want to use as your default calendar."
If the default there looks right, try creating an event and check to see which calendar it is being added to. Try disabling that calendar from Settings > Calendar > Accounts > [Account name]. Then, test again by adding a new event. It could be that this is related to a particular account.
Take care.