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This is my guess as well. In my experience, most "good" implementations directly use the certificate chain issued by Let's Encrypt during the certificate-acquisition process rather than storing or "pinning" intermediates, which leads to problems like this.
https://letsencrypt.org/certs/2024/r11.pem
Here in this community we strongly discourage using third-party services like this due to their potentially-questionable implementations that could violate Let's Encrypt's subscriber agreement due to the way account and certificate keys are handled (not to mention lack of automation). Even for "one-off" situations, I recommend using a vetted ACME client (I author 
CertSage as an example).