CSV files exported from hosted feature layers in ArcGIS Enterprise portal are encoded in UTF-8 without byte order mark (BOM).

Solução Provisória

1. Open the .CSV in Notepad++.
2. Under the Encoding drop-down menu, switch the encoding of the file from UTF-8 to UTF-8-BOM, and save the file.
3. Open the .CSV in Microsoft Excel. 

The Thai characters appear as expected.

Etapas para Reproduzir