KSP fails with “couldn't make a guess for mypackage.MyGlideModule”

After switching from kapt to KSP I'm getting this error while building:

[ksp] java.lang.IllegalArgumentException: couldn't make a guess for com.ubergeek42.WeechatAndroid.media.WAGlideModule

Execution failed for task ':app:kspDebugKotlin'.
> A failure occurred while executing com.google.devtools.ksp.gradle.KspAAWorkerAction
   > couldn't make a guess for com.ubergeek42.WeechatAndroid.media.WAGlideModule

The configuration is as follows:

// root
plugins {
    id("com.google.devtools.ksp") version "2.3.3" apply false
}

// app
plugins {
    id("com.google.devtools.ksp")
}

dependencies {
    implementation("com.github.bumptech.glide:glide:5.0.5")
    ksp("com.github.bumptech.glide:ksp:5.0.5")
}

Also tried KSP 2.2.21-2.0.4 and Glide 4.16.0. Using Kotlin 2.2.21. The actual class can be found here.

The documentation says that AppGlideModule is not deprecated, and I don't think I'm using any other deprecated APIs anywhere. kapt works fine. KSP with Room works fine. What could be wrong here?

(I should note that I haven't touched the Glide code in a long time and that I know little of KSP besides that it's supposed to be the new shiny thing. So this very well might be not a problem with Glide and something else entirely, and just maybe very silly.)