Customer define Flow action using addErrorMessage() to display custom error message.

=========================
Process Automation > Flow Designer > New > Action
Name : jw_1006_AddErrorMsg

Script
--------------
(function execute(inputs, outputs) {
gs.addErrorMessage(inputs.message);
})(inputs, outputs);
--------------
=========================

When customer call this FD action from business rule directly, custom error message is displayed as expected.
However, when they call this FD action from BR via subflow, custom error message is not displayed.

Affected instance(s): empjwp1
Affected version(s): Build tag: glide-paris-06-24-2020__patch1-hotfix1-09-15-2020

1.Create Flow Action using addErrorMessage(), then publish
Process Automation > Flow Designer > New > Action

Name : jw_1006_AddErrorMsg
Accessible From : All application scopes
Application : Global

Inputs
Label | Name | Type | Mandatory
-----------------------------------------
Message | message | String | Yes

Script step
Input Variables
Name | Value
-----------------------------
message | action > Message

Script
--------------
(function execute(inputs, outputs) {
// ... code ...
gs.addErrorMessage(inputs.message);
})(inputs, outputs);
--------------

2.Create Business Rule to call FD Action
System Definition > Business Rules

Name : jw_TestBR01
Table : Incident
Active : Yes
Advanced : Yes

When to run
When : before
Order : 10
Insert : Yes
Update : Yes
Filter Condition
Short description contains TestBR01

Advanced
Condition : (null)
Script
----------------------
(function executeRule(current, previous /*null when async*/ ) {
var inputs = {};
inputs['message'] = gs.getMessage('Test Message From jw_TestBR01'); // String
sn_fd.FlowAPI.executeAction('global.jw_1006_AddErrorMsg', inputs);

current.setAbortAction(true);
current.setWorkflow(false);
})(current, previous);
----------------------

3.Test to create a incident
Incident > Create New
Number : INC0010002
Short description : TestBR01
Save
==>
Test Message From jw_TestBR01
Invalid insert

The custom error message is displayed with "Invalid insert" as expected.
BR:jw_TestBR01 and FD Action:jw_1006_AddErrorMsg works normally.

4.Create Subflow to call FD Action
Process Automation > Flow Designer > New > Subflow

Name : jw_1006_Subflow_01
Application : Global
Accessible From : This application scope only
Run As : User who initiates session

Inputs
Label | Name | Type | Mandatory
---------------------------------------------------
03_sys_id | 03_sys_id | String | Yes
04_err_message | 04_err_message | String | Yes

Outputs
Label | Name | Type
-------------------------
result | result | String

Actions
1:jw_1006_AddErrorMsg
Action : jw_1006_AddErrorMsg
Message : Input > 04_err_message

2:Assign Subflow Outputs
Name | Data
----------------
result | NG

5.Create Business Rule to call Subflow
System Definition > Business Rules

Name : jw_TestBR02
Table : Incident
Active : Yes
Advanced : Yes

When to run
When : before
Order : 11
Insert : Yes
Update : Yes
Filter Condition
Short description contains TestBR02

Advanced
Condition : (null)
Script
----------------------
(function executeRule(current, previous /*null when async*/ ) {

var inputs = {};
var sys_id_03 = current.sys_id;

inputs['03_sys_id'] = sys_id_03; // String
inputs['04_err_message'] = gs.getMessage('Test Message From jw_TestBR02'); // String

var result = sn_fd.FlowAPI.getRunner().subflow('global.jw_1006_Subflow_01').inForeground().withInputs(inputs).run();
var outputs = result.getOutputs();

var answer =outputs['result'];

if(answer === 'NG'){
current.setAbortAction(true);
current.setWorkflow(false);
}

})(current, previous);
----------------------

6.Test to create a incident
Incident > Create New
Number : INC0010003 (Sys ID:59f4eff71b6fd810811c520d0a4bcb5a)
Short description : TestBR02
Save
==>
Invalid insert

The custom error message is NOT displayed, only "Invalid insert".

7.Check Flow engine context
Process Automation > Flow Administration > Today's Execution

Created : 2020-10-07 00:07:53
Name : jw_1006_Subflow_01
State : Complete
Runtime : 3
Created by : admin
Sys Id : 2725e33b316fd81075d1d5914dca84df
https://empjwp1.service-now.com/nav_to.do?uri=sys_flow_context.do?sys_id=2725e33b316fd81075d1d5914dca84df

Open in Operations View
https://empjwp1.service-now.com/$flow-designer.do#/operations/context/2725e33b316fd81075d1d5914dca84df

Subflow Input
VARIABLE NAME | TYPE | RUNTIME VALUE
------------------------------------------------------------
03_sys_id | String | 59f4eff71b6fd810811c520d0a4bcb5a
04_err_message | String | Test Message From jw_TestBR02

Subflow Output
VARIABLE NAME | TYPE | RUNTIME VALUE
----------------------------------------
result | String | NG

==>
Subflow set inputs correctly, but custom errror message is not displayed
In this situation, FD Action:jw_1006_AddErrorMsg does not work as expected.

Investigation Summary:
Customer said this issue is not occurred before upgrading from Orlando to Paris.
But when I tried to reproduce the issue using subflow in Orlando OOB(empjwo1), incident updated with no message, both custom message and "Invalid insert" are not displayed.

Create a subflow output with the message to be displayed or status code. 

Pass this output back to the calling business rule who should really display the message anyway