the explanation of leftmost first and then longest will be valid even if I didn't use named groups. So, this behavior is generic.

Yes, perlre says:

In Perl the groups are numbered sequentially regardless of being named or not. Thus in the pattern

/(x)(?<foo>y)(z)/
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$+{foo} will be the same as $2, and $3 will contain 'z'

And the description of (?|pattern) says:

Named captures are implemented as being aliases to numbered groups holding the captures

Also, I remember finding the description of the left-to-right operation of the regex engine in the Camel quite enlightening.