Setting the Alarm Clock's Bluetooth volume I have LG G3 with CyanogenMod 13 (Android 6.0) and a Bluetooth speaker.  

In the phone settings I have chosen maximum alarm volume. When the alarm rings through the phone's internal loudspeaker the volume is maximum (which I can get from the phone's loudspeaker).  

When the alarm clock rings and my phone is connected to the Bluetooth speakers the volume of the Bluetooth speakers is not at its maximum (in fact the volume of the Bluetooth speakers is at the level it was left when listening to music).

Is this a common issue on Android 6.0 (are other users having this issue as well or am I the "only one")?
You need to adjust the volume of the alarm once you connect your BT speakers. In Android, the volume of audio is determined by two mostly orthogonal things: the first is the type of the audio (it's called stream type), so you have separate volume settings for media, ringtones, alarms, etc; the second is the kind of the output device: built-in speaker, headphones, BT headset, USB, etc.

You might have noticed that media volume for the phone speaker and headphones is controlled separately. The same thing is for BT sound output.