The issue that your comment about cached/discovered intermediates relates to is that some servers were still manually constructing the chain that goes leaf -> R3 -> DST Root X3, and the R3 intermediate signed by X3 expired on 29 September. This chain hasn't been returned by Let's Encrypt since May 2021.
The other issue relates to the current default chain returned by Let's Encrypt that goes leaf -> R3 -> ISRG Root X1 -> DST Root X3 for Android compatibility. Most clients are able to successfully build a valid chain from the leaf to the still valid ISRG X1, however, old versions of OpenSSL (pre-1.1.0) and several other TLS libraries that don't explore the graph correctly barfs at a chain that terminates in the now expired X3 root.
[1]: https://community.letsencrypt.org/t/production-chain-changes...