I had a situation where I used syspref
KohaAdminEmailAddress:     "The Library" <thelibrary@example.com>
and my overdue notices went out fine.

Then at a later point the branch details email field had
thelibrary@example.com
and was put in the overdue notice  as   <<branches.branchemail>>
and the notice generated fine.
BUT
the overdue notice From: now became  thelibrary@example.com
which wasn't as pretty.
THEN
I put   "The Library" <thelibrary@example.com>
in the branch details email field
BUT the notice showed:   "The Library"
as <thelibrary@example.com> got gobbled up by the Letters template code.
It thought it was a stragling  <tag> that had no substitution.

Workaround:
a) blank out  branch details email field, so KohaAdminEmailAddress is used.
b) replace  <<branches.branchemail>>   witht he literal branch email address.

Just providing a workaround.