Default sandbox config missing in UI5 serve

Expected Behavior

After setting up SAPUI5 version 1.81.0 and running UI5 serve I should be able to serve the same files belonging to SAPUI5 as the content delivery server for all declared libraries so meaning http://localhost:8080/test-resources/... should bring the same results as https://sapui5.hana.ondemand.com/1.81.0/test-resources/...

Current Behavior

http://localhost:8080/test-resources/sap/ushell/shells/sandbox/fioriSandboxConfig.json
brings a 404: Cannot GET /test-resources/sap/ushell/shells/sandbox/fioriSandboxConfig.json
whereby
https://sapui5.hana.ondemand.com/1.81.0/test-resources/sap/ushell/shells/sandbox/fioriSandboxConfig.json brings the correct file.

As a result I cannot run my App in sandbox mode because it relies on the default sandbox configuration which cannot be loaded.
The sandbox itself under http://localhost:8080/test-resources/sap/ushell/bootstrap/sandbox.js and other SAPUI5 files are loaded fine so it seems my general setup is working.

Steps to reproduce the issue

1. Prepare a ui5.yaml file:

specVersion: "2.2"
type: application
metadata:
  name: [...]
framework:
  name: SAPUI5
  version: 1.81.0
  libraries:
    - name: sap.ui.core
    - name: sap.m
    - name: sap.ushell
      development: true
    - name: themelib_sap_fiori_3

2. UI5 serve
3. Open URL http://localhost:8080/test-resources/sap/ushell/shells/sandbox/fioriSandboxConfig.json in browser

Context

• UI5 Module Version (output of ui5 --version when using the CLI): 2.4.1
• Node.js Version: v11.11.0
• npm Version: 6.7.0
• OS/Platform: Windows 10 1909 x64
• Browser (if relevant): Chrome 84

Affected components (if known)

• ui5-builder
• ui5-server
• ui5-cli
• ui5-fs
• ui5-project
• ui5-logger

Log Output / Stack Trace

info normalizer:translators:ui5Framework Using SAPUI5 version: 1.81.0
Server started
URL: http://localhost:8080

The following stack trace when trying to open my app in sandbox mode in browser:

[...]
2020-08-26 16:37:15.817814 Mixing/Overwriting sandbox configuration from http://localhost:8080/test-resources/sap/ushell/bootstrap/../shells/sandbox/fioriSandboxConfig.json. -  
jquery.js?eval:9275 GET http://localhost:8080/test-resources/sap/ushell/shells/sandbox/fioriSandboxConfig.json 404 (Not Found)
send @ jquery.js?eval:9275
ajax @ jquery.js?eval:8756
sjax @ jquery.sap.sjax.js?eval:73
applyJsonApplicationConfig @ sandbox.js:141
bootstrap @ sandbox.js:215
constructor @ Core.js?eval:549
eval @ Core.js?eval:4231
(anonymous) @ ui5loader.js:1826
requireAll @ ui5loader.js:1749
executeModuleDefinition @ ui5loader.js:1800
ui5Define @ ui5loader.js:1932
eval @ Core.js?eval:8
execModule @ ui5loader.js:1664
requireModule @ ui5loader.js:1550
requireSync @ ui5loader.js:2059
(anonymous) @ flpSandbox.html:13
[...]