What LetsEncrypt Client did you use to obtain the expired certificates?
As others have noted above, the R3 certificate expiring on 9/29/2021 was replaced earlier this year; the replacement is also called R3 but has a future expiry which is past the DST expiry (9/30/2021). Most browsers and devices will accept this, because they will short-circuit the validation to the ISRG-X1 root certificate that signed the identical keypair as R3.
Because of short-circuiting, how SSL clients build chains, and the presence of cross-signed certificates, devices/browsers will not reliably show the actual certificates used during a connection - they often just show (or use) the first valid certificates that match a key pairing.
So why do we care about LetsEncrypt clients?
Many clients have ignored the RFC and hardcoded the certificate chains, so when using them you will download a valid certificate from LetsEncrypt but it will be chained against a potentially invalid certificate bundled into the client's source code. This will still work in production for many situations if the keypairs are the same (as in R3-old and R3-new) and the client can build a valid path, but analyzing the chain locally can generate errors.