FirebaseApiNotAvailableException when trying to import stickers to GBoard with uSticker

When trying to import stickers from a directory in uSticker, the process fails with this error:

com.google.firebase.FirebaseApiNotAvailableException: API: AppIndexing.API is not available on this device.
	at com.google.firebase.FirebaseExceptionMapper.getException(Unknown Source:3)
	at com.google.android.gms.common.api.internal.zag.zaa(Unknown Source:9)
	at com.google.android.gms.common.api.internal.GoogleApiManager$zaa.zac(Unknown Source:9)
	at com.google.android.gms.common.api.internal.GoogleApiManager$zaa.onConnectionFailed(Unknown Source:32)
	at com.google.android.gms.common.api.internal.zabo.run(Unknown Source:23)
	at android.os.Handler.handleCallback(Handler.java:873)
	at android.os.Handler.dispatchMessage(Handler.java:99)
	at com.google.android.gms.internal.base.zap.dispatchMessage(Unknown Source:1)
	at android.os.Looper.loop(Looper.java:193)
	at android.os.HandlerThread.run(HandlerThread.java:65)

uSticker: https://github.com/apsun/uSticker
Relevant issue in that repository: https://github.com/apsun/uSticker/issues/18

LineageOS 16 (Pie), MicroG installed through Nanondroid with patched playstore.
Everything ticked in self-check except for Network-based location enabled.
Device registration, GCM, SafetyNet all enabled.
MicroG Core version: 0.2.10.19420

Start by reproducing the uSticker directory import on LineageOS 16 with MicroG Core 0.2.10.19420, then compare the failure with uSticker issue 18 and trace the AppIndexing.API availability path. Done means sticker import completes without the FirebaseApiNotAvailableException on the reported setup.