BUG: Chained mutate operations give wrong results

[ayushi04]

Mutate function is giving wrong output. Please check below sample example.

table_expr = ibis.table([('a', 'string'), ('b', 'string')]) 
table_expr1 = table_expr['b'].fillna('Short Term') 
table_expr = table_expr.mutate(b = table_expr1) 
table_expr2 = table_expr['a'].fillna('Short Term') 
table_expr = table_expr.mutate(a = table_expr2) 
print(ibis.impala.compile(table_expr)) 

For the above program output is :
SELECT isnull(a, 'Short Term') AS a, b FROM t0

instead of
SELECT isnull(a, 'Short Term') AS a, isnull(b, 'Short Term') AS b FROM t0

Let me know if anything is wrong in my understanding.