Support adaptive icons (Android 8, API>=26)
See https://f-droid.org/packages/de.schildbach.wallet/
Or https://f-droid.org/packages/de.schildbach.wallet_test/
I guess it's due to the fact the icon is an adaptive icon. If you would resolve the icon as an API level 25 (Android 7.1) or below client, you'd still get the old PNG icon though (mipmap/ic_app_color_48dp).
Edited by username-removed-151489