Whenever I submit a response for my form I receive the default confirmation message even though I have changed it in the settings. It automatically goes to the default message.
DEBUG --> GFFormDisplay::handle_confirmation(): Invalid confirmation; using default text instead.
That logging statement is found when the theme or another plugin is using the gform_confirmation filter, and they didn’t return the correct value.
If you aren’t sure which theme or plugin includes that code, you might need to run through a conflict test using the steps outlined on this documentation page: Testing for a Theme/Plugin Conflict?