The following three ABC code samples should be equivalent:

X:0         X:0            X:0
K:C bass    K:Cmaj bass    K:C major bass
A,,B,,C,    A,,B,,C,       A,,B,,C,

However, the first does not render correctly.

X:0
K:C bass
A,,B,,C,

Whereas the other two both render correctly, as follows.

X:0
K:C major bass
A,,B,,C,

EDIT: The incorrect rendering appears to be platform/browser specific. On Win10/Chrome85.0.4183.102, I see

However on Win10/Edge and on my iPad, I see the correct rendering.