Why does Chrome use sec-ch-ua: "\"Not\\A;Brand";v="99"?
It seems that it's part of Chromium's GREASEing strategy:
User agents' brands containing more than a single entry could encourage standardized processing of the UA string. By randomly including additional, intentionally incorrect, comma-separated entries with arbitrary ordering, they would reduce the chance that we ossify on a few required strings.
Looking at the Chromium repository, it seems that it was introduced in this commit
The commit description given is:
[client-hints] GREASEing the Sec-CH-UA listRandomizing order and string with escaped characters to ensure properparsing and prevent ossification.It also links to this ticket in the bug tracker.