Insecured notice when visiting site sometimes

I have a website behind the Cloudflare proxy. Both the apex domain and www subdomain are proxied. SSL mode is set to "Full (strict)". "HSTS" (12 months, include subdomains, preload) and "Always Use HTTPS" is enabled under the edge certificates. The domain has a pro subscription. However, sometimes, our client sees the "nortvi.com doesn't support encrypted connections" message in Chrome. They even asked ChatGPT (I know) and it also seems to be able to detect the missing certificate. This resolved itself pretty quickly. This happened a few times and I have never been able to reproduce this myself or have had this on any other zone. Do y'all have ANY clue what might be happening here and/or how to debug / resolve this?