Hello @giannishadj,
Welcome to the Klaviyo Community!
Since Convert Plus is a third-party app that has developed their own integration with Klaviyo, I would suggest reaching out to their developers and support team for more details on this issue.
From my experience though, I would suggest checking the double opt-in setting of your Klaviyo list as subscribers would still need to adhere to this setting prior to being added to the Klaviyo list. This means that if double opt-in is enabled, users who subscribe through your Convert Plus form would still need to confirm their opt-in confirmation email sent from Klaviyo prior to being added to the list.
I hope this helps!
David