I attempt to second try with another example:
PCRE2 version 10.33 2019-04-16
/(?<=(?<=a)b)c.*/info
Capture group count = 0
Max lookbehind = 1
First code unit = 'c'
Subject length lower bound = 1
abc\=ph
Partial match: bc
<
Why max lookbehind=1, but not 2?