Using Nextcloud 15.0.5
If theming app disabled, favicon-touch is working and accessible (e.g. /nextcloud/core/img/favicon-touch.png)
If theming app enabled, no favicon-touch (touchIcon) is created by theming app.
data/appdata_xxx/theming/63/touchIcon-files is created but with no content.
Access error in browser, when using /nextcloud/apps/theming/icon/settings?v=63 (Content not valid)
In the nextcloud log I found following errors
[PHP] Error: filesize(): stat failed for data/appdata_xxx/theming/63/touchIcon-files at nextcloud/lib/private/LargeFileHelper.php#171
[PHP] Error: stat(): stat failed for data/appdata_xxx/theming/63/touchIcon-files at nextcloud/lib/private/Files/Storage/Local.php#142
[PHP] Error: file_get_contents(data/appdata_xxx/theming/63/touchIcon-files): failed to open stream: No such file or directory at nextcloud/lib/private/Files/Storage/Local.php#216