It sounds like you're experiencing a frustrating drop in your Klaviyo signup form’s submit rate. Given that no changes were made to the form or the traffic source, yet a different form with a similar design outperformed it, there could be several underlying issues at play.

1. Tracking and Metrics: First, ensure that the tracking for your Klaviyo form is functioning correctly. Sometimes, a technical issue can cause discrepancies in how submissions are counted or reported. Review the form metrics in Klaviyo's dashboard to spot any irregularities in form views, engagement, or errors.

2. Form Fatigue or Overexposure: It’s possible that your audience has become overexposed to the form, leading to "form fatigue." Consider rotating or refreshing the design, offer, or targeting settings to recapture attention. Experiment with A/B testing to find what resonates better with your audience.

3. Technical Glitches or Site Speed: Even though you didn't change the form, small updates to your website or issues like slow loading times can impact form performance. It's worth checking that the Klaviyo form is loading promptly and correctly across all devices.

4. Popup Timing and Triggers: If your Klaviyo form uses time-based triggers or exit-intent popups, compare these settings with those used in the Converflow form. Even subtle differences in when and how the form appears can significantly affect conversion rates.

To troubleshoot further, here are some helpful articles:

• Troubleshooting decreasing KPIs
• Troubleshooting sign-up forms

These resources should help you diagnose and address the issue, hopefully getting your submission rates back up.