The issue is not reproducible.
For cross-platform support, it is recommended to use an Arcade expression.
When using an Arcade expression, the layer’s REST resources are displayed as follows:
"labelingInfo": [
{"labelExpressionInfo": {
"expression": "$feature.STATE_NAME + \" (\" + $feature.STATE_FIPS + \")\"",
"title": "my expression"
}}]
When a Python expression is used, it is translated into a simpler form, as shown below. This is because Python expressions are not supported by web client apps.
"labelingInfo": [
{
"labelExpression": "CONCAT [STATE_NAME] CONCAT \"(\" CONCAT [STATE_FIPS] CONCAT \")\""
}]