Reusing scripted field in another scripted field as a parameter in painless

[anoymous_cooper]

I am trying to use a scripted field as a parameter in another scripted field but it seems to give wrong result in es7.4.2,It was working fine in es5.0.1

"script_fields": {
"a": {
"script": {
"source": "return 1",
"lang": "painless"
},
"ignore_failure": false
},
"b": {
"script": {
"source": "if(params.val != null){return params.val}",
"lang": "painless",
"params": {
"val":"a"
}
},
"ignore_failure": false
}

expected output -> value of field "a" is "1" and "b" is also "1"
but actual result ->value of field "a" is 1 and "b" is "a"
can anyone tell me what is wrong with the query