Why is Flutter not using the correct Kotlin version?

Question

I am getting the following errors when running flutter build apk in Windows (using VSCode)

The binary version of its metadata is 1.8.0, expected version is 1.6.0.

However, in my build.gradle, I have:

buildscript {
    ext.kotlin_version = '1.7.10'
    repositories {
        google()
        mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:7.2.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'com.google.gms:google-services:4.3.15'

        classpath 'com.google.firebase:firebase-crashlytics-gradle:2.8.1'
    }
}

Really confused! Why is it not using Kotlin 1.7.10 (error suggests it's using 1.6.0?) and why is it then trying to use libraries that require 1.8.0?

EDIT:

I should have said that I did try the obvious top question related to this on Stack before asking this question:

Module was compiled with an incompatible version of Kotlin. The binary version of its metadata is 1.5.1, expected version is 1.1.15

And I did try the top-rated (non-accepted) answer to this:

https://stackoverflow.com/a/74425347/187030

But this has not solved the problem for me.

I would also say this used to work (I have all the prerequisites installed from here, and flutter doctor is all green ticks)

I am probably doing something really n00by here as I'm a Windows .Net developer and I appreciate Flutter is wrapping a toolchain that I am not at all familiar with... but I'd really like to understand why I'm getting this problem and how to diagnose a solution for it....

Also, worth saying it does create the APK despite the errors, and, weirdly, if I build the apk with debug, then I don't get these errors:

flutter build apk <-- builds with errors ❌

flutter build apk --debug <-- builds without errors ✅

flutter build appbundle <-- builds without errors ✅