Just knowing that g = 8 doesn't allow you to infer that g is a function or to conclude anything about DOMAIN g, so TLAPS does the right thing. (For example, 42 denotes some unspecified value in TLA+, would you want to conclude that 42 is a function?) You want to state "type predicates" for the functions that appear in your specification – typically as ASSUME clauses for constant parameters, and as invariants for variables.
ASSUME A1 == f = [x \in DOMAIN f |-> f[x]]
ASSUME A2 == f = 8