Keep "Let's Encrypt Authority X3" for renewed certificate

Not exactly.
It should never authorize a cert beyond its' own life.
So it had to stop issuing 90 day certs... 91 days before it expires.
[2021/03/17 - 91 days was on 2020/12/16]
As also seen on the first line of this post: Beginning Issuance from R3 - API Announcements - Let's Encrypt Community Support