probably depends on how you have the pickups adjusted. Most strat pickups have staggered pole pieces and the highs are much lower than the mids, then if you have the bass side adjusted higher than the treble side you could have the situation you describe.
But if the pickups are adjusted to be level, even if they have the staggered pole pieces, you should not have an all bassy tone left as you turn down the volume. Possible bad volume pot or a better one needs to be installed IMO. There will be many that can help you better though so wait for some more responses
Welcome to theFret!
Dave
Guitars: Ibanez AF-75, Schecter Solo-6 Custom, Douglas SG
Amps: Fender Princeton 65, Marshall AVT50
Pedals: Metal Muff, MXR Smart Gate, EHX Cathedral Reverb, Digitech RP-255