CA not appearing under Squid's SSL filtering dropdown

[T145]

I've created an ACME certificate for the router and internal network systems so Let's Encrypt has been added as a CA. However it doesn't appear as an option when selecting a CA for Squid's SSL filtering. Having configured the proxy as transparent, my assumption is that selecting the Let's Encrypt CA will automatically handle certificates so they wouldn't need to be installed on each system in the network. Is this reasoning accurate? Are there steps I'm missing?

EDIT: The Let's Encrypt CA does appear under the general "Extra Trusted CA" option dropdown.