I’ve found a solution for mine! Such a silly bug. If you have Klayvio or another email app, navigate to the opt in settings - For Klayvio - lists and segments - newsletter - settings - consent. Have both Shopify AND this page open and select single opt in on both pages. Worked for me. Hope this helps..
Topic summary
Merchants report that even with the double opt-in setting disabled, new subscribers still receive a confirmation email (double opt-in = a follow-up email requiring a click to confirm subscription). One user shared a screenshot showing Double Opt-in set to OFF, yet a test order still triggered the confirmation message.
Scope: Multiple stores are affected, with shop URLs provided. A helper requested URLs and a Loom/YouTube capture to diagnose but later advised contacting Shopify Support, suggesting it may be outside theme/developer control.
Workaround reported: For stores using Klaviyo or similar email apps, also switch the list to single opt-in in the app (Klaviyo path: Lists & Segments > Newsletter list > Settings > Consent). One participant said setting single opt-in in both Shopify and Klaviyo—while having both pages open—resolved the issue for them.
Open questions: Another user asked what “have both Shopify AND this page open” specifically means and how it impacts Shopify settings; no clarification provided. No official fix or root cause was confirmed; the discussion remains partially unresolved.