Update / clarification: I realized my original post listed the wrong versions.
I’m actually on Spring Boot 3.5.3 with Java 21.
For reference, here’s the relevant part of my build.gradle

plugins {
    id 'java'
    id 'org.springframework.boot' version '3.5.3'
    id 'io.spring.dependency-management' version '1.1.7'
}

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(21)
    }
}

The rest of the question remains the same—just wanted to correct the environment details.