Legacy Migration Incompatibility - Why does MessageDigestPasswordEncoder consider '{}' to be part of the salt?

Summary

I'm upgrading from Spring Security 4.x to 5.x.

The ReflectionSaltSource from Spring 4 lets us configure a custom salt. But that's removed in Spring Security 5. I then found out that I should use MessageDigestPasswordEncoder. It has a detailed java-doc but unfortunately the doc is like a bag of words without conveying any structured information (I tried multiple times; my bad if I was ignorant).

Anyways I thought I should do the following based on my limited understanding of the javadoc.

Old system with 4.x - myEncodedPassword and mySalt are passed separately to the encoder.

New System with 5.x - Pass one field with the value {mySalt}myEncodedPassword to the MessageDigestPasswordEncoder

Actual Behavior

The Problem was that when MessageDigestPasswordEncoder sees {mySalt}encodedPassword, it uses {mySalt} (with the {}) as the salt instead of using mySalt as the salt . I'm confused.

Expected Behavior

I'd expect the salt to be mySalt and not {mySalt}

Version

group='org.springframework.security', module='spring-security-core', version='5.1.4.RELEASE'

Sample

I've used Groovy to create a concise sample.

@Grab(group='org.springframework.security', module='spring-security-core', version='5.1.4.RELEASE')
import org.springframework.security.crypto.password.MessageDigestPasswordEncoder

String password = 'myPassword'
String salt_1 = 'mySalt'
String salt_2 = '{mySalt}'
// http://www.lorem-ipsum.co.uk/hasher.php generated below hashes for above salt and password combinations
String encodedPasswordWithSalt_1 = '57bc828628811a10496215e217b7ae9b714c859fc7a8b1c678c9a0cc40aac422'
String encodedPasswordWithSalt_2 = 'a18b53fc58843def1e08e00a718f40d6f8eda0b97ef97824b5078c1fad93c0c5'

MessageDigestPasswordEncoder encoder = new MessageDigestPasswordEncoder('SHA-256')

// expected to match but did not
println "expected=true, actual=" + encoder.matches(password, "{${salt_1}}${encodedPasswordWithSalt_1}") 

// why does this match?
println "expected=false, actual=" + encoder.matches(password, "{${salt_1}}${encodedPasswordWithSalt_2}") 

No source file or test is named. Start by running the provided Groovy sample with Spring Security 5.1.4 and inspect MessageDigestPasswordEncoder's documented input format and implementation. Done means establishing whether braces are intentional and, if not, defining a focused regression test and the expected salt-handling behavior.