Thanks for the reply, that seems something different than my problem, but good to know 
Have a nice day.
Topic summary
Main issue: After switching from Mollie to Shopify Payments, a sharp rise in conversions attributed to direct/none (~5% to ~50%) appears in Google Analytics. Many conversions show landing pages like /checkout/processing with very short sessions, suggesting source is lost during the payment flow.
Investigation steps and findings:
- Segments/User Explorer confirm checkout as first page and too-short durations to reflect real sessions.
- Referrals via hooks.stripe.com; excluding it increases direct attribution.
- Possible cross-domain tracking lapse (GA client ID handoff, 2-minute window). Attempts to use GA linker in Additional GA JS were tested but inconclusive; Shopify can’t assist on cross-domain parameters.
- GA4 considered/tried; no native Shopify integration and poor implementations reported; some see the issue worsen in GA4.
- GTM is the favored fix but effectively limited to Shopify Plus; third-party apps (e.g., Elevar) suggested.
Additional causes/notes:
- Apple Pay/Google Pay and cookie consent (GDPR) apps can break attribution; GTM + Cookiebot on Shopify Plus mitigates the issue. Residual direct may come from in-app browser switches, cart emails without UTMs, or misconfigured gateways.
Status: Unresolved for non-Plus stores. Shopify acknowledges the issue but has had it “under investigation” for 6+ months with no fix or clear guidance.