Regardless of whether or not you provide your own SSL certificates, cloudflare still uses their own between their servers and client browsers. So any SSL encrypted traffic is unencrypted at their end before being re-encrypted with your certificate. How can such an entity be trusted?
From my observations, my certificate is used between cloudflare and my server and another cloudflare issued certificate is provided to the client’s web-browser.
In other words, traffic between the browser and CF servers use a CF certificate, then traffic between CF server and my server use my own certificate.
Another way of putting it is that when I host my site directly, the browser reports the certificate as being generated by LetsEncrypt (by me).
However, when I add CF to the equation, the browser shows cloudflare as the certificate creator.