Good day!
It seems last code unit "c" is not detected and so start optimization
don't work:
PCRE2 version 10.34-RC1 2019-04-22
/\Aabc/info,auto_callout
Capture group count = 0
Max lookbehind = 1
Compile options: auto_callout
Overall options: anchored auto_callout
First code unit = 'a'
Subject length lower bound = 3
abx
--->abx
+0 ^ \A
+2 ^ a
+3 ^^ b
+4 ^ ^ c
No match