It may be a fault with the speaker or may just be you used a different dynamic range device.
All devices boost each frequency in a slightly different ways so some boost bass more, some boost treble more so you may find, as you describe, the bass is louder than you are accustomed to which makes it appear variable volume based on the frequency spread.
If you want to ask support then email email@example.com , or wait a while for other Soundcore 2 owners to show up with their feedback.
I own the Soundcore XL, Mini, Nano and some buds and to my ears I prefer the Nano, others prefer the Mini and others the XL. The XL particularly boosts bass so a quiet bass on one is a louder bass on another.