Can having a commercial buffer tube on a mil spec lower cause malfunctions?
Not really - mil-spec vs commercial relates to the outside diameter of the tubes. Milspec is slightly smaller than commercial.
That’s what I mean though I have a commercial buffer tube on a milspec lower
Doesnt matter, the outside diameter is the only thing that is different, you have to use a different stock because of this.
The inside diameter is the same, uses the same spring and buffer.
Won’t cause malfunctions, although a lot of commercial end plates are not properly staked, which can cause the tube to become loose.