Reproduction steps:
1. Open the project from the attached file "1241343.zip"
2. In the toolbar go to Custom > TextTest
3. From Assets open the newly created test.txt file
4. Inspect the content of the file

Expected result: File contains: "hello world"
Actual result: File contains:
"%YAML 1.1
%TAG !u! tag:unity3d.com,2011:
--- !u!49 &4900000
TextAsset:
m_ObjectHideFlags: 0
m_CorrespondingSourceObject: {fileID: 0}
m_PrefabInstance: {fileID: 0}
m_PrefabAsset: {fileID: 0}
m_Name: test
m_Script: hello world"

Reproducible with: 2018.4.23f1, 2019.3.14f1, 2020.1.0b9, 2020.2.0a11

Notes: Reproducible with Asset Pipeline v1 and v2