Can only submit my Sprout Form form if I'm logged into my site
This might not help but I've had a similar problem with a couple of my own modules and then with LInkVault that I went around in circles with. It suddenly stopped allowing non logged in users from downloading files. I contacted support who spotted the issue and fixed it in minutes. They gave this message:

Link Vault’s FrontEndController class has the allowAnonymous property set to true so that can’t be part of the problem

then a few minutes later:

Apparently, a recent Craft update added some logic in the craft\web\Controller::init() method that converts boolean allowAnonymous values to an integer which was part of the check that determines whether guests should be able to access a controller action. Link Vault’s front end controller had its own init() method but it wasn’t calling the parent’s init() method. So that is fixed now.

I leave this here in case it's any help either to this specific problem or anyone else with modules that run in to this issue.