Kimball (1991: 101) does make reference to a defective verb meaning 'to be a member of a group; to hold an office; to be something', which lacks 3rd person forms, but it appears to be in complementary distribution with a root meaning 'it is the case that', which is used only in 3rd person. So this is probably better treated as an instance of suppletion.
Kimball, Geoffrey. 1991. Koasati grammar. Lincoln: University of Nebraska Press.