Actually, after some testing, I don't think it is entirely about how the volume is controlled.
Here's a list of my devices and how they behave (and this is true on iOS 11.4 thru 12.0.0)
BEEP BETWEEN SONGS
Soundpeats Q800 headphones (BT 4.0)
- beep with volume change, whether changed on phone or on headphones
- volume on phone linked to volume on headphones
Taotronics BT headphones (TT-BH07) (BT 4.1)
- beep with volume change, whether changed on phone or on headphones
- volume on phone linked to volume on headphones
DOES NOT BEEP BETWEEN SONGS
Plantroics Explorer 50 BT Headset (BT 3.0)
- beep with volume change
- volume independent from phone. Volume change on phone makes no beep.
ANKER Soundcore Mini (BT 4.0) and Soundcore 2 (BT 4.2) - both behave same way
- NO beep with volume change
- volume linked to phone volume
2014 BMW
- NO beep with volume change
- volume independent from phone. normally have phone volume at max and control volume with car.
Based on that the problem appears to be independent of BT version but related to whether your headphones are tied to your phone's volume and beep at you to indicate volume changes. It is as though the headphones are detecting a volume change when the songs change.