Understanding DST Root expiry and the older default cert chain

[rg305]

@psykid

0: Subject: CN = *.stackexchange.com
   Issuer:  C = US, O = Let's Encrypt, CN = R3
   Not after:    Mar  3 14:00:52 2021 GMT
1: Subject: C = US, O = Let's Encrypt, CN = R3
   Issuer:  O = Digital Signature Trust Co., CN = DST Root CA X3
   Not after:    Sep 29 19:21:40 2021 GMT

They are serving the wrong chain.
That chain has expired and hasn't been provided since May 2021.
There is nothing you can do on the client side to fix such a mistake.